Discussion

What are your final thoughts on this book (positive, negative, or both)? Write a blurb summarizing the entire book’s main ideas in 1 to 5 sentences. Finally, pose a question you would like a classmate to answer about the book?

“The town of El Valle de Antón, in central Panama, sits in the middle of a volcanic crater formed about a million years ago. The crater is almost four miles wide, but when the weather is clear you can see the jagged hills that surround the town like the walls of a ruined tower. El Valle has one main street, a police station, and an open-air market. In addition to the usual assortment of Panama hats and vividly colored embroidery, the market offers what must be the world’s largest selection of golden-frog figurines. There are golden frogs resting on leaves and golden frogs sitting up on their haunches and—rather more difficult to understand—golden frogs clasping cell phones. There are golden frogs wearing frilly skirts and golden frogs striking dance poses and golden frogs smoking cigarettes through a holder, after the fashion of FDR. The golden frog, which is taxicab yellow with dark brown splotches, is endemic to the area around El Valle. It is considered a lucky symbol in Panama; its image is (or at least used to be) printed on lottery tickets.

”

“As recently as a decade ago, golden frogs were easy to spot in the hills around El Valle. The frogs are toxic—it’s been calculated that the poison contained in the skin of just one animal could kill a thousand average-sized mice—hence the vivid color, which makes them stand out against the forest floor. One creek not far from El Valle was nicknamed Thousand Frog Stream. A person walking along it would see so many golden frogs sunning themselves on the banks that, as one herpetologist who made the trip many times put it to me, “it was insane—absolutely insane.”

Then the frogs around El Valle started to disappear. The problem—it was not yet perceived as a crisis—was first noticed to the west, near Panama’s border with Costa Rica. An American graduate student happened to be studying frogs in the rainforest there. She went back to “the States for a while to write her dissertation, and when she returned, she couldn’t find any frogs or, for that matter, amphibians of any kind. She had no idea what was going on, but since she needed frogs for her research, she set up a new study site, farther east. At first the frogs at the new site seemed healthy; then the same thing happened: the amphibians vanished. The blight spread through the rainforest until, in 2002, the frogs in the hills and streams around the town of Santa Fe, about fifty miles west of El Valle, were effectively wiped out. In 2004, little corpses began showing up even closer to El Valle, around the town of El Copé. By this point, a group of biologists, some from Panama, others from the United States, had concluded that the golden frog was in grave danger. They decided to try to preserve a remnant population by removing a few dozen of each sex from the forest and raising them indoors. But whatever was killing the frogs was moving even faster than the biologists had feared. Before they could act on their plan, the wave hit.”

“I first read about the frogs of El Valle in a nature magazine for children that I picked up from my kids. The article, which was illustrated with full-color photos of the Panamanian golden frog and other brilliantly colored species, told the story of the spreading scourge and the biologists’ efforts to get out in front of it. The biologists had hoped to have a new lab facility constructed in El Valle, but it was not ready in time. They raced to save as many animals as possible, even though they had nowhere to keep them. So what did they end up doing? They put them “in a frog hotel, of course!” The “incredible frog hotel”—really a local bed and breakfast—agreed to let the frogs stay (in their tanks) in a block of rented rooms.

“With biologists at their beck and call, the frogs enjoyed first-class accommodations that included maid and room service,” the article noted. The frogs were also served delicious, fresh meals—“so fresh, in fact, the food could hop right off the plate.

“Just a few weeks after I read about the “incredible frog hotel,” I ran across another frog-related article written in a rather different key. This one, which appeared in the Proceedings of the National Academy of Sciences, was by a pair of herpetologists. It was titled “Are We in the Midst of the Sixth Mass Extinction? A View from the World of Amphibians.” The authors, David Wake, of the University of California-Berkeley, and Vance Vredenburg, of San Francisco State, noted that there “have been five great mass extinctions during the history of life on this planet.” These extinctions they described as events that led to “a profound loss of biodiversity.” The first took place during the late Ordovician period, some 450 million years ago, “when living things were still mainly confined to the water. The most devastating took place at the end of the Permian period, some 250 million years ago, and it came perilously close to emptying the earth out altogether. (This event is sometimes referred to as “the mother of mass extinctions” or “the great dying.”) The most recent—and famous—mass extinction came at the close of the Cretaceous period; it wiped out, in addition to the dinosaurs, the plesiosaurs, the mosasaurs, the ammonites, and the pterosaurs. Wake and Vredenburg argued that, based on extinction rates among amphibians, an event of a similarly catastrophic nature was currently under way. Their article was illustrated with just one photograph, of about a dozen mountain yellow-legged frogs—all dead—lying bloated and belly-up on some rocks.

“I understood why a kids’ magazine had opted to publish photos of live frogs rather than dead ones. I also understood the impulse to play up the Beatrix Potter–like charms of amphibians ordering room service. Still, it seemed to me, as a journalist, that the magazine had buried the lede. Any event that has occurred just five times since the first animal with a backbone appeared, some five hundred million years ago, must qualify as exceedingly rare. The notion that a sixth such event would be taking place right now, more or less in front of our eyes, struck me as, to use the technical term, mind-boggling. Surely this story, too—the bigger, darker, far more consequential one—deserved telling. If Wake and Vredenburg were correct, then those of us alive today not only are witnessing one of the rarest events in life’s history, we are also causing it. “One weedy species,” the pair observed, “has unwittingly achieved the ability to directly affect its own fate and that of most of the other species on this planet.” A few days after I read Wake and Vredenburg’s article, I booked a ticket to Panama.”

“THE El Valle Amphibian Conservation Center, or EVACC (pronounced “ee-vac”), lies along a dirt road not far from the open-air market where the golden frog figurines are sold. It’s about the size of a suburban ranch house, and it occupies the back corner of a small, sleepy zoo, just beyond a cage of very sleepy sloths. The entire building is filled with tanks. There are tanks lined up against the walls and more tanks stacked at the center of the room, like books on the shelves of a library. The taller tanks are occupied by species like the lemur tree frog, which lives in the forest canopy; the shorter tanks serve for species like the big-headed robber frog, which lives on the forest floor. Tanks of horned marsupial frogs, which carry their eggs in a pouch, sit next to tanks of casque-headed frogs, which carry their eggs on their backs. A few dozen tanks are devoted to Panamanian golden frogs, Atelopus zeteki.”

“Golden frogs have a distinctive, ambling gait that makes them look a bit like drunks trying to walk a straight line. They have long, skinny limbs, pointy yellow snouts, and very dark eyes, through which they seem to be regarding the world warily. At the risk of sounding weak-minded, I will say that they look intelligent. In the wild, females lay their eggs in shallow running water; males, meanwhile, defend their territory from the tops of mossy rocks. In EVACC, each golden frog tank has its own running water, provided by its own little hose, so that the animals can breed near a simulacrum of the streams that were once their home. In one of the ersatz streams, I noticed a string of little pearl-like eggs. On a white board nearby someone had noted excitedly that one of the frogs “depositó huevos!!”

“EVACC sits more or less in the middle of the golden frog’s range, but it is, by design, entirely cut off from the outside world. Nothing comes into the building that has not been thoroughly disinfected, including the frogs, which, in order to gain entry, must first be treated with a solution of bleach. Human visitors are required to wear special shoes and to leave behind any bags or knapsacks or equipment that they’ve used out in the field. All of the water that enters the tanks has been filtered and specially treated. The sealed-off nature of the place gives it the feel of a submarine or, perhaps more aptly, an ark mid-deluge.

”

“EVACC’s director is a Panamanian named Edgardo Griffith. Griffith is tall and broad-shouldered, with a round face and a wide smile. He wears a silver ring in each ear and has a large tattoo of a toad’s skeleton on his left shin. Now in his mid-thirties, Griffith has devoted pretty much his entire adult life to the amphibians of El Valle, and he has turned his wife, an American who came to Panama as a Peace Corps volunteer, into a frog person, too. Griffith was the first person to notice when little carcasses started showing up in the area, and he personally collected many of the several hundred amphibians that got booked into the hotel. (The animals were transferred to EVACC once the building had been completed.) If EVACC is a sort of ark, Griffith becomes its Noah, though one on extended duty, since already he’s been at things a good deal longer than forty days. Griffith told me that a key part of his job was getting to know the frogs as individuals. “Every one of them has the same value to me as an elephant,” he said.”

“The first time I visited EVACC, Griffith pointed out to me the representatives of species that are now extinct in the wild. These included, in addition to the Panamanian golden frog, the Rabbs’ fringe-limbed tree frog, which was first identified only in 2005. At the time of my visit, EVACC was down to just one Rabbs’ frog, so the possibility of saving even a single, Noachian pair had obviously passed. The frog, greenish brown with yellow speckles, was about four inches long, with oversized feet that gave it the look of a gawky teenager. Rabbs’ fringe-limbed tree frogs lived in the forest above El Valle, and they laid their eggs in tree holes. In an unusual, perhaps even unique arrangement, the male frogs cared for the tadpoles by allowing their young, quite literally, to eat the skin off their backs. Griffith said that he thought there were probably many other amphibian species that had been missed in the initial collecting rush for EVACC and had since vanished; it was hard to say how many, since most of them were probably unknown to science. “Unfortunately,” he told me, “we are losing all these amphibians before we even know that they exist.””

“WHEN the first reports that frog populations were crashing began to circulate, a few decades ago, some of the most knowledgeable people in the field were the most skeptical. Amphibians are, after all, among the planet’s great survivors. The ancestors of today’s frogs crawled out of the water some 400 million years ago, and by 250 million years ago the earliest representatives of what would become the modern amphibian orders—one includes frogs and toads, the second newts and salamanders, and the third weird limbless creatures called caecilians—had evolved. This means that amphibians have been around not just longer than mammals, say, or birds; they have been around since before there were dinosaurs.

”

“Most amphibians—the word comes from the Greek meaning “double life”—are still closely tied to the aquatic realm from which they emerged. (The ancient Egyptians thought that frogs were produced by the coupling of land and water during the annual flooding of the Nile.) Their eggs, which have no shells, must be kept moist in order to develop. There are many frogs that, like the Panamanian golden frog, lay their eggs in streams. There are also frogs that lay them in temporary pools, frogs that lay them underground, and frogs that lay them in nests that they construct out of foam. In addition to frogs that carry their eggs on their backs and in pouches, there are frogs that carry them wrapped like bandages around their legs. Until recently, when both of them went extinct, there were two species of frogs, known as gastric-brooding frogs, that carried their eggs in their stomachs and gave birth to little froglets through their mouths.”

“Amphibians emerged at a time when all the land on earth was part of a single expanse known as Pangaea. Since the breakup of Pangaea, they’ve adapted to conditions on every continent except Antarctica. Worldwide, just over seven thousand species have been identified, and while the greatest number are found in the tropical rainforests, there are occasional amphibians, like the sandhill frog of Australia, that can live in the desert, and also amphibians, like the wood frog, that can live above the Arctic Circle. Several common North American frogs, including spring peepers, are able to survive the winter frozen solid, like popsicles. Their extended evolutionary history means that even groups of amphibians that, from a human perspective, seem to be fairly similar may, genetically speaking, be as different from one another as, say, bats are from horses.”

“David Wake, one of the authors of the article that sent me to Panama, was among those who initially did not believe that amphibians were disappearing. This was back in the mid–nineteen-eighties. Wake’s students began returning from frog-collecting trips in the Sierra Nevada empty-handed. Wake remembered from his own student days, in the nineteen-sixties, that frogs in the Sierras had been difficult to avoid. “You’d be walking through meadows, and you’d inadvertently step on them,” he told me. “They were just everywhere.” Wake assumed that his students were going to the wrong spots, or that they just didn’t know how to look. Then a postdoc with several years of collecting experience told him that he couldn’t find any amphibians, either. “I said, ‘OK, I’ll go up with you, and we’ll go out to some proven places,’” Wake recalled. “And I took him out to this proven place, and we found like two toads.”

“Part of what made the situation so mystifying was the geography; frogs seemed to be vanishing not only from populated and disturbed areas but also from relatively pristine places, like the Sierras and the mountains of Central America. In the late nineteen-eighties, an American herpetologist went to the Monteverde Cloud Forest Reserve in northern Costa Rica to study the reproductive habits of golden toads. She spent two field seasons looking; where once the toads had mated in writhing masses, a single male was sighted. (The golden toad, now classified as extinct, was actually a bright tangerine color. It was only very distantly related to the Panamanian golden frog, which, owing to a pair of glands located behind its eyes, is also technically a toad.) Around the same time, in central Costa Rica, biologists noticed that the populations of several endemic frog species had crashed. Rare and highly specialized species were vanishing and so, too, were much more familiar ones. In Ecuador, the Jambato toad, a frequent visitor to backyard gardens, disappeared in a matter of years. And in northeastern Australia the southern day frog, once one of the most common in the region, could no longer be found.”

“The first clue to the mysterious killer that was claiming frogs from Queensland to California came—perhaps ironically, perhaps not—from a zoo. The National Zoo, in Washington, D.C., had been successfully raising blue poison-dart frogs, which are native to Suriname, through many generations. Then, more or less from one day to the next, the zoo’s tank-bred frogs started dropping. A veterinary pathologist at the zoo took some samples from the dead frogs and ran them through an electron scanning microscope. He found a strange microorganism on the animals’ skin, which he eventually identified as a fungus belonging to a group known as chytrids”

“Chytrid fungi are nearly ubiquitous; they can be found at the tops of trees and also deep underground. This particular species, though, had never been seen before; indeed, it was so unusual that an entire genus had to be created to accommodate it. It was named Batrachochytrium dendrobatidis—batrachos is Greek for “frog”—or Bd for short.”

“The veterinary pathologist sent samples from infected frogs at the National Zoo to a mycologist at the University of Maine. The mycologist grew cultures of the fungus and then sent some of them back to Washington. When healthy blue poison-dart frogs were exposed to the lab-raised Bd, they sickened. Within three weeks, they were dead. Subsequent research showed that Bd interferes with frogs’ ability to take up critical electrolytes through their skin. This causes them to suffer what is, in effect, a heart attack.”

“EVACC can perhaps best be described as a work-in-progress. The week I spent at the center, a team of American volunteers was also there, helping to construct an exhibit. The exhibit was going to be open to the public, so, for biosecurity purposes, the space had to be isolated and equipped with its own separate entrance. There were holes in the walls where, eventually, glass cases were to be mounted, and around the holes someone had painted a mountain landscape very much like what you would see if you stepped outside and looked up at the hills. The highlight of the exhibit was to be a large case full of Panamanian golden frogs, and the volunteers were trying to construct a three-foot-high concrete waterfall for them. But there were problems with the pumping system and difficulties getting replacement parts in a valley with no hardware store. The volunteers seemed to be spending a lot of time hanging around, waiting.”

“I spent a lot of time hanging around with them. Like Griffith, all of the volunteers were frog lovers. Several, I learned, were zookeepers who worked with amphibians back in the States. (One told me that frogs had ruined his marriage.) I was moved by the team’s dedication, which was the same sort of commitment that had gotten the frogs into the “frog hotel” and then had gotten EVACC up and running, if not entirely completed. But I couldn’t help also feeling that there was also something awfully sad about the painted green hills and the fake waterfall.”

“With almost no frogs left in the forests around El Valle, the case for bringing the animals into EVACC has by now clearly been proved. And yet the longer the frogs spend in the center, the tougher it is to explain what they’re doing there. The chytrid fungus, it turns out, does not need amphibians in order to survive. This means that even after it has killed off the animals in an area, it continues to live on, doing whatever it is that chytrid fungi do. Thus, were the golden frogs at EVACC allowed to amble back into the actual hills around El Valle, they would sicken and collapse. (Though the fungus can be destroyed by bleach, it’s obviously impossible to disinfect an entire rainforest.) Everyone I spoke to at EVACC told me that the center’s goal was to maintain the animals until they could be released to repopulate the forests, and everyone also acknowledged that they couldn’t imagine how this would actually be done.”

“We’ve got to hope that somehow it’s all going to come together,” Paul Crump, a herpetologist from the Houston Zoo who was directing the stalled waterfall project, told me. “We’ve got to hope that something will happen, and we’ll be able to piece it all together, and it will all be as it once was, which now that I say it out loud sounds kind of stupid.”

“The point is to be able to take them back, which every day I see more like a fantasy,” Griffith said.

Once chytrid swept through El Valle, it didn’t stop; it continued to move east. It has also since arrived in Panama from the opposite direction, out of Colombia. Bd has spread through the highlands of South America and down the eastern coast of Australia, and it has crossed into New Zealand and Tasmania. It has raced through the Caribbean and has been detected in Italy, Spain, Switzerland, and France. In the U.S., it appears to have radiated from several points, not so much in a wavelike pattern as in a series of ripples. At this point, it appears to be, for all intents and purposes, unstoppable.”

“THE same way acoustical engineers speak of “background noise” biologists talk about “background extinction.” In ordinary times—times here understood to mean whole geologic epochs—extinction takes place only very rarely, more rarely even than speciation, and it occurs at what’s known as the background extinction rate. This rate varies from one group of organisms to another; often it’s expressed in terms of extinctions per million species-years. Calculating the background extinction rate is a laborious task that entails combing through whole databases’ worth of fossils. For what’s probably the best-studied group, which is mammals, it’s been reckoned to be roughly .25 per million species-years. This means that, since there are about fifty-five hundred mammal species wandering around today, at the background extinction rate you’d expect—once again, very roughly—one species to disappear every seven hundred years.”

“Mass extinctions are different. Instead of a background hum there’s a crash, and disappearance rates spike. Anthony Hallam and Paul Wignall, British paleontologists who have written extensively on the subject, define mass extinctions as events that eliminate a “significant proportion of the world’s biota in a geologically insignificant amount of time.” Another expert, David Jablonski, characterizes mass extinctions as “substantial biodiversity losses” that occur rapidly and are “global in extent.” Michael Benton, a paleontologist who has studied the end-Permian extinction, uses the metaphor of the tree of life: “During a mass extinction, vast swathes of the tree are cut short, as if attacked by crazed, axe-wielding madmen.” A fifth paleontologist, David Raup, has tried looking at matters from the perspective of the victims: “Species are at a low risk of extinction most of the time.” But this “condition of relative safety is punctuated at rare intervals by a vastly higher risk.” The history of life thus consists of “long periods of boredom interrupted occasionally by panic.”

”

“The Big Five extinctions, as seen in the marine fossil record, resulted in a sharp decline in diversity at the family level. If even one species from a family made it through, the family counts as a survivor, so on the species level the losses were far greater.

In times of panic, whole groups of once-dominant organisms can disappear or be relegated to secondary roles, almost as if the globe has undergone a cast change. Such wholesale losses have led paleontologists to surmise that during mass extinction events—in addition to the so-called Big Five, there have been many lesser such events—the usual rules of survival are suspended. Conditions change so drastically or so suddenly (or so drastically and so suddenly) that evolutionary history counts for little. Indeed, the very traits that have been most useful for dealing with ordinary threats may turn out, under such extraordinary circumstances, to be fatal.”

“A rigorous calculation of the background extinction rate for amphibians has not been performed, in part because amphibian fossils are so rare. Almost certainly, though, the rate is lower than it is for mammals. Probably, one amphibian species should go extinct every thousand years or so. That species could be from Africa or from Asia or from Australia. In other words, the odds of an individual’s witnessing such an event should be effectively zero. Already, Griffith has observed several amphibian extinctions. Pretty much every herpetologist working out in the field has watched several. (Even I, in the time I spent researching this book, encountered one species that has since gone extinct and three or four others, like the Panamanian golden frog, that are now extinct in the wild.) “I sought a career in herpetology because I enjoy working with animals,” Joseph Mendelson, a herpetologist at Zoo Atlanta, has written. “I did not anticipate that it would come to resemble paleontology.”

“Today, amphibians enjoy the dubious distinction of being the world’s most endangered class of animals; it’s been calculated that the group’s extinction rate could be as much as forty-five thousand times higher than the background rate. But extinction rates among many other groups are approaching amphibian levels. It is estimated that one-third of all reef-building corals, a third of all freshwater mollusks, a third of sharks and rays, a quarter of all mammals, a fifth of all reptiles, and a sixth of all birds are headed toward oblivion. The losses are occurring all over: in the South Pacific and in the North Atlantic, in the Arctic and the Sahel, in lakes and on islands, on mountaintops and in valleys. If you know how to look, you can probably find signs of the current extinction event in your own backyard.”

“There are all sorts of seemingly disparate reasons that species are disappearing. But trace the process far enough and inevitably you are led to the same culprit: “one weedy species.”

“Bd is capable of moving on its own. The fungus generates microscopic spores with long, skinny tails; these propel themselves through water and can be carried far longer distances by streams, or in the runoff after a rainstorm. (It’s likely this sort of dispersal produced what showed up in Panama as an eastward-moving scourge.) But this kind of movement cannot explain the emergence of the fungus in so many distant parts of the world—Central America, South America, North America, Australia—more or less simultaneously. One theory has it that Bd was moved around the globe with shipments of African clawed frogs, which were used in the nineteen-fifties and sixties in pregnancy tests. (Female African clawed frogs, when injected with the urine of a pregnant woman, lay eggs within a few hours.) Suggestively, African clawed frogs do not seem to be adversely affected by Bd, though they are widely infected with it. A second theory holds that the fungus was spread by North American bullfrogs which have been introduced—sometimes accidentally, sometimes purposefully—into Europe, Asia, and South America, and which are often exported for human consumption. North American bullfrogs, too, are widely infected with Bd but “do not seem to be harmed by it. The first has become known as the “Out of Africa” and the second might be called the “frog-leg soup” hypothesis.”

“Either way, the etiology is the same. Without being loaded by someone onto a boat or a plane, it would have been impossible for a frog carrying Bd to get from Africa to Australia or from North America to Europe. This sort of intercontinental reshuffling, which nowadays we find totally unremarkable, is probably unprecedented in the three-and-a-half-billion-year history of life.

”

“EVEN though Bd has swept through most of Panama by now, Griffith still occasionally goes out collecting for EVACC, looking for survivors. I scheduled my visit to coincide with one of these collecting trips, and one evening I set out with him and two of the American volunteers who were working on the waterfall. We headed east, across the Panama Canal, and spent the night in a region known as Cerro Azul, in a guesthouse ringed by an eight-foot-tall iron fence. At dawn, we drove to the ranger station at the entrance to Chagres National Park. Griffith was hoping to find females of two species that EVACC is short of. He pulled out his government-issued collecting permit and presented it to the sleepy officials manning the station. Some underfed dogs came out to sniff around the truck.”

“Beyond the ranger station, the road turned into a series of craters connected by deep ruts. Griffith put Jimi Hendrix on the truck’s CD player, and we bounced along to the throbbing beat. Frog collecting requires a lot of supplies, so Griffith had hired two men to help with the carrying. At the very last cluster of houses, in the tiny village of Los Ángeles, the men materialized out of the mist. We bounced on until the truck couldn’t go any farther; then we all got out and started to walk.”

“The trail wound its way through the rainforest in a slather of red mud. Every few hundred yards, the main path was crossed by a narrower one; these paths had been made by leaf-cutter ants, making millions—perhaps billions—of trips to bring bits of greenery back to their colonies. (The colonies, which look like mounds of sawdust, can cover an area the size of a city park.) One of the Americans, Chris Bednarski, from the Houston Zoo, warned me to avoid the soldier ants, which will leave their jaws in your shin even after they’re dead. “Those’ll really mess you up,” he observed. The other American, John Chastain, from the Toledo Zoo, was carrying a long hook, for use against venomous snakes. “Fortunately, the ones that can really mess you up are pretty rare,” “Bednarski assured me. Howler monkeys screamed in the distance. Griffith pointed out jaguar prints in the soft ground.”

“After about an hour, we came to a farm that someone had carved out of the trees. There was some scraggly corn growing, but no one was around, and it was hard to say whether the farmer had given up on the poor rainforest soil or was simply away for the day. A flock of emerald green parrots shot up into the air. After another several hours, we emerged into a small clearing. A blue morpho butterfly flitted by, its wings the color of the sky. There was a small cabin on the site, but it was so broken down that everyone elected to sleep outside. Griffith helped me string up my bed—a cross between a tent and a hammock that had to be hung between two trees. A slit in the bottom constituted the entryway, and the top was supposed to provide protection against the inevitable rain. When I climbed into the thing, I felt as if I were lying in a coffin.”

“That evening, Griffith prepared some rice on a portable gas burner. Then we strapped on headlamps and clambered down to a nearby stream. Many amphibians are nocturnal, and the only way to see them is to go looking in the dark, an exercise that’s as tricky as it sounds. I kept slipping, and violating Rule No. 1 of rainforest safety: never grab onto something if you don’t know what it is. After one of my falls, Bednarski pointed out to me a tarantula the size of my fist sitting on the next tree over.”

“Practiced hunters can find frogs at night by shining a light into the forest and looking for the reflected glow of their eyes. The first amphibian Griffith sighted this way was a San Jose Cochran frog, perched on top of a leaf. San Jose Cochran frogs are part of a larger family known as “glass frogs,” so named because their translucent skin reveals the outline of their internal organs. This particular glass frog was green, with tiny yellow dots. Griffith pulled a pair of surgical gloves out of his pack. He stood completely still and then, with a heronlike gesture, darted to scoop up the frog. With his free hand, he took what looked like the end of a Q-tip and swabbed the frog’s belly. He put the Q-tip in a little plastic vial—it would later be sent to a lab and analyzed for Bd—and since it wasn’t one of the species he was looking for, he placed the frog back on the leaf. Then he pulled out his camera. The frog stared back at the lens impassively.”

“We continued to grope through the blackness. Someone spotted a La Loma robber frog, which is orangey-red, like the forest floor; someone else spotted a Warzewitsch frog, which is bright green and shaped like a leaf. With every animal, Griffith went through the same routine: snatching it up, swabbing its belly, photographing it. ”

“Finally, we came upon a pair of Panamanian robber frogs locked in amplexus—the amphibian version of sex. Griffith left these two alone.”

“One of the amphibians that Griffith was hoping to catch, the horned marsupial frog, has a distinctive call that’s been likened to the sound of a champagne bottle being uncorked. As we sloshed along—by this point we were walking in the middle of the stream—we heard the call, which seemed to be emanating from several directions at once. At first, it sounded as if it were right nearby, but as we approached, it seemed to get farther away. Griffith began imitating the call, making a cork-popping sound with his lips. Eventually, he decided that the rest of us were scaring the frogs with our splashing. He waded ahead, and we stayed for a long time up to our knees in water, trying not to move. When Griffith finally gestured us over, we found him standing in front of a large yellow frog with long toes and an owlish face. It was sitting on a tree limb, just above eye level. Griffith was looking to find a female horned marsupial frog to add to EVACC’s collection. He shot out his arm, grabbed the frog, and flipped it over. Where a female horned marsupial would have a pouch “this one had none. Griffith swabbed it, photographed it, and placed it back in the tree.”

“You are a beautiful boy,” he murmured to the frog.

Around midnight, we headed back to camp. The only animals that Griffith decided to bring with him were two tiny blue-bellied poison frogs and one whitish salamander, whose species neither he nor the two Americans could identify. The frogs and the salamander were placed in plastic bags with some leaves to keep them moist. It occurred to me that the frogs and their progeny, if they had any, and their progeny’s progeny, if they had any, would never again touch the floor of the rainforest but would live out their days in disinfected glass tanks. That night it poured, and in my coffinlike hammock I had vivid, troubled dreams, the only scene from which I could later recall was of a bright yellow frog smoking a cigarette through a holder.”

“Extinction may be the first scientific idea that kids today have to grapple with. One-year-olds are given toy dinosaurs to play with, and two-year-olds understand, in a vague sort of way at least, that these small plastic creatures represent very large animals. If they’re quick learners—or, alternatively, slow toilet trainers—children still in diapers can explain that there were once lots of kinds of dinosaurs and that they all died off long ago. (My own sons, as toddlers, used to spend hours over a set of dinosaurs that could be arranged on a plastic mat depicting a forest from the Jurassic or Cretaceous. The scene featured a lava-spewing volcano, which, when you pressed on it, emitted a delightfully terrifying roar.) All of which is to say that extinction strikes us as an obvious idea. It isn’t.”

“Aristotle wrote a ten-book History of Animals without ever considering the possibility that animals actually had a history. Pliny’s Natural History includes descriptions of animals that are real and descriptions of animals that are fabulous, but no descriptions of animals that are extinct. The idea did not crop up during the Middle Ages or during the Renaissance, when the word “fossil” was used to refer to anything dug up from the ground (hence the term “fossil fuels”). In the Enlightenment, the prevailing view was that every species was a link in a great, unbreakable “chain of being.” As Alexander Pope put it in his Essay on Man:

All are but parts of one stupendous whole,

Whose body nature is, and God the soul.”

“When Carl Linnaeus introduced his system of binomial nomenclature, he made no distinction between the living and the dead because, in his view, none was required. The tenth edition of his Systema Naturae, published in 1758, lists sixty-three species of scarab beetle, thirty-four species of cone snail, and sixteen species of flat fishes. And yet in the Systema Naturae, there is really only one kind of animal—those that exist.”

“This view persisted despite a sizable body of evidence to the contrary. Cabinets of curiosities in London, Paris, and Berlin were filled with traces of strange creatures that no one had ever seen—the remains of animals that would now be identified as trilobites, belemnites, and ammonites. Some of the last were so large their fossilized shells approached the size of wagon wheels. In the eighteenth century, mammoth bones increasingly made their way to Europe from Siberia. These, too, were shoehorned into the system. The bones looked a lot like those of elephants. Since there clearly were no elephants in contemporary Russia, it was decided that they must have belonged to beasts that had been washed north in the great flood of Genesis.”

“Extinction finally emerged as a concept, probably not coincidentally, in revolutionary France. It did so largely thanks to one animal, the creature now called the American mastodon, or Mammut americanum, and one man, the naturalist Jean-Léopold-Nicolas-Frédéric Cuvier, known after a dead brother simply as Georges. Cuvier is an equivocal figure in the history of science. He was far ahead of his contemporaries yet also held many of them back; he could be charming and he could be vicious; he was a visionary and, at the same time, a reactionary. By the middle of the nineteenth century, many of his ideas had been discredited. But the most recent discoveries have tended to support those very theories of his that were most thoroughly vilified, with the result that Cuvier’s essentially tragic vision of earth history has come to seem prophetic.”

“WHEN, exactly, Europeans first stumbled upon the bones of an American mastodon is unclear. An isolated molar unearthed in a field in upstate New York was sent off to London in 1705; it was labeled the “tooth of a Giant.” The first mastodon bones subjected to what might, anachronistically, be called scientific study were discovered in 1739. That year, Charles le Moyne, the second Baron de Longueuil, was traveling down the Ohio River with four hundred troops, some, like him, Frenchmen, most of the others Algonquians and Iroquois. The journey was arduous and supplies were short. On one leg, a French soldier would later recall, the troops were reduced to living off acorns. Sometime probably in the fall, Longueuil and his troops set up camp on the east bank of the Ohio, not far from what is now the city of Cincinnati. Several of the Native Americans set off to go hunting.”

“A few miles away, they came to a patch of marsh that gave off a sulfurous smell. Buffalo tracks led to the marsh from all directions, and hundreds—perhaps thousands—of huge bones poked out of the muck, like spars of a ruined ship. The men returned to camp carrying a thigh bone three and a half feet long, an immense tusk, and several huge teeth. The teeth had roots the length of a human hand, and each one weighed nearly ten pounds.”

“Longueuil was so intrigued by the bones that he instructed his troops to take them along when they broke camp. Lugging the enormous tusk, femur, and molars, the men pushed on through the wilderness. Eventually, they reached the Mississippi River, where they met up with a second contingent of French troops. Over the next several months, many of Longueuil’s men died of disease, and the campaign they had come to wage, against the Chickasaw, ended in humiliation and defeat. Nevertheless, Longueuil kept the strange bones safe. He made his way to New Orleans and from there shipped the tusk, the teeth, and the giant femur to France. They were presented to Louis XV, who installed them in his museum, the Cabinet du Roi. Decades later, maps of the Ohio River valley were still largely blank, except for the Endroit où on a trouvé des os d’Éléphant—the “place where the elephant bones were found.” (Today the “place where the elephant bones were found” is a state park in Kentucky known as Big Bone Lick.)”

“Longueuil’s bones confounded everyone who examined them. The femur and the tusk looked as if they could have belonged to an elephant or, much the same thing according to the taxonomy of the time, a mammoth. But the animal’s teeth were a conundrum. They resisted categorization. Elephants’ teeth (and also mammoths’) are flat on top, with thin ridges that run from side to side, so that the chewing surface resembles the sole of a running shoe. Mastodon teeth, by contrast, are cusped. They do, indeed, look as if they might belong to a jumbo-sized human. The first naturalist to study one of them, Jean-Étienne Guettard, declined even to guess at its provenance.

“What animal does it come from?” he asked plaintively in a paper delivered to France’s Royal Academy of Sciences in 1752.”

“In 1762, the keeper of the king’s cabinet, Louis-Jean-Marie Daubenton, tried to resolve the puzzle of the curious teeth by declaring that the “unknown animal of the Ohio” was not an animal at all. Rather, it was two animals. The tusks and leg bones belonged to elephants; the molars came from another creature entirely. Probably, he decided, this other creature was a hippopotamus.”

“Right around this time, a second shipment of mastodon bones was sent to Europe, this time to London. These remains, also from Big Bone Lick, exhibited the same befuddling pattern: the bones and tusks were elephant-like, while the molars were covered in knobby points. William Hunter, attending physician to the queen, found Daubenton’s explanation for the discrepancy wanting. He offered a different explanation—the first halfway accurate one.

“The supposed American elephant,” he submitted, was a totally new animal with “which anatomists were unacquainted.” It was, he decided, carnivorous, hence its scary-looking teeth. He dubbed the beast the American incognitum.”

“France’s leading naturalist, Georges-Louis Leclerc, Comte de Buffon, added yet another twist to the debate. He argued that the remains in question represented not one or two, but three separate animals: an elephant, a hippopotamus, and a third, as-yet-unknown species. With great trepidation, Buffon allowed that this last species—“the largest of them all”—seemed to have disappeared. It was, he proposed, the only land animal ever to have done so.”

“In 1781, Thomas Jefferson was drawn into the controversy. In his Notes on the State of Virginia, written just after he left the state’s governorship, Jefferson concocted his own version of the incognitum. The animal was, he maintained with Buffon, the largest of all beasts—“five or six times the cubic volume of the elephant.” (This would disprove the theory, popular in Europe at the time, that the animals of the New World were smaller and more “degenerate” than those of the Old.) The creature, Jefferson agreed with Hunter, was probably carnivorous. But it was still out there somewhere. If it could not be found in Virginia, it was roaming those parts of the continent that “remain in their aboriginal state, unexplored and undisturbed.” When, as president, he dispatched Meriwether Lewis and William Clark to the Northwest, Jefferson hoped that they would come upon live incognita roaming its forests.”

“Such is the economy of nature,” he wrote, “that no instance can be produced of her having permitted any one race of her animals to become extinct; of her having formed any link in her great work so weak as to be broken.”

CUVIER arrived in Paris in early 1795, half a century after the remains from the Ohio Valley had reached the city. He was twenty-five years old, with wide-set gray eyes, a prominent nose, and a temperament one friend compared to the exterior of the earth—generally cool but capable of violent tremors and eruptions.” “Cuvier had grown up in a small town on the Swiss border and had few contacts in the capital. Nevertheless, he had managed to secure a prestigious position there, thanks to the passing of the ancien régime on the one hand and his own sublime self-regard on the other. An older colleague would later describe him as popping up in Paris “like a mushroom.”

Cuvier’s job at Paris’s Museum of Natural History—the democratic successor to the king’s cabinet—was, officially, to teach. But in his spare time, he delved into the museum’s collection. He spent long hours studying the bones that Longueuil had sent to Louis XV, comparing them with other specimens. On April 4, 1796—or, according to the revolutionary calendar in use at the time, 15 Germinal Year IV—he presented the results of his research at a public lecture.”

“Cuvier’s job at Paris’s Museum of Natural History—the democratic successor to the king’s cabinet—was, officially, to teach. But in his spare time, he delved into the museum’s collection. He spent long hours studying the bones that Longueuil had sent to Louis XV, comparing them with other specimens. On April 4, 1796—or, according to the revolutionary calendar in use at the time, 15 Germinal Year IV—he presented the results of his research at a public lecture.”

“Cuvier began by discussing elephants. Europeans had known for a long time that there were elephants in Africa, which were considered dangerous, and elephants that resided in Asia, which were said to be more docile. Still, elephants were regarded as elephants, much as dogs were dogs, some gentle and others ferocious. On the basis of his examination of the elephant remains at the museum, including one particularly well-preserved skull from Ceylon and another from the Cape of Good Hope, Cuvier had recognized—correctly, of course—that the two belonged to separate species.”

“It is clear that the elephant from Ceylon differs more from that of Africa than the horse from the ass or the goat from the sheep,” he declared. Among the animals’ many distinguishing characteristics were their teeth. The elephant from Ceylon had molars with wavy ridges on the surface “like festooned ribbons,” while the elephant from the Cape of Good Hope had teeth with ridges arranged in the shape of diamonds. Looking at live animals would not have revealed this difference, as who would have the temerity to peer down an elephant’s throat? “It is to anatomy alone that zoology owes this interesting discovery,” Cuvier declared.”

“Having successfully, as it were, sliced the elephant in two, Cuvier continued with his dissection. The accepted theory about the giant bones from Russia, Cuvier concluded after “scrupulous examination” of the evidence, was wrong. The teeth and jaws from Siberia “do not exactly resemble those of an elephant.” They belonged to another species entirely. As for the teeth of the animal from Ohio, well, a single glance was “sufficient to see that they differ still further.”

“What has become of these two enormous animals of which one no longer finds any living traces?” he asked. The question, in Cuvier’s formulation, answered itself. They were espèces perdues, or lost species. Already, Cuvier had doubled the number of extinct vertebrates, from (possibly) one to two. He was just getting going.”

“A few months earlier, Cuvier had received sketches of a skeleton that had been discovered on the bank of the Río Luján, west of Buenos Aires. The skeleton—twelve feet long and six feet high—had been shipped to Madrid, where it had been painstakingly reassembled. Working from the sketches, Cuvier had identified its owner—once again, correctly—as some sort of outlandishly oversized sloth. He named it Megatherium, meaning “giant beast.” Though he had never traveled to Argentina, or, for that matter, anywhere farther than Germany, Cuvier was convinced that Megatherium was no longer to be found lumbering along the rivers of South America. It, too, had disappeared. The same was true of the so-called Maastricht animal, whose remains—an enormous, pointy jaw studded with sharklike teeth—had been found in a Dutch

“quarry. (The Maastricht fossil had recently been seized by the French, who occupied the Netherlands in 1795.)

And if there were four extinct species, Cuvier declared, there must be others. The proposal was a daring one to make given the available evidence. On the basis of a few scattered bones, Cuvier had conceived of a whole new way of looking at life. Species died out. This was not an isolated but a widespread phenomenon.

“All these facts, consistent among themselves, and not opposed by any report, seem to me to prove the existence of a world previous to ours,” Cuvier said. “But what was this primitive earth? And what revolution was able to wipe it out?”

“SINCE Cuvier’s day, the Museum of Natural History has grown into a sprawling institution with outposts all over France. Its main buildings, though, still occupy the site of the old royal gardens in the Fifth Arrondissement. Cuvier didn’t just work at the museum; for most of his adulthood, he also lived on the grounds, in a large stucco house that has since been converted into office space. Next to the house, there’s now a restaurant and next to that a menagerie, where, on the day that I visited, some wallabies were sunning themselves on the grass. Across the gardens, there’s a large hall that houses the museum’s paleontology collection.”

“Pascal Tassy is a director at the museum who specializes in proboscideans, the group that includes elephants and their lost cousins—mammoths, mastodons, and gomphotheres, to name just a few. I went to visit him because he’d promised to take me to see the very bones Cuvier had handled. I found Tassy in his dimly lit office, in the basement under the paleontology hall, sitting amid a mortuary’s worth of old skulls. The walls of the office were decorated with covers from old Tintin comic books. Tassy told me he’d decided to become a paleontologist when he was seven, after reading a Tintin adventure about a dig.”

“We chatted about proboscideans for a while. “They’re a fascinating group,” he told me. “For instance, the trunk, which is a change of anatomy in the facial area that is truly extraordinary, it evolved separately five times. Two times—yes, that’s surprising. But it happened five times independently! We are forced to accept this by looking at the fossils.” So far, Tassy said, some 170 proboscidean species have been identified, going back some fifty-five million years, “and this is far from complete, I am sure.”

We headed upstairs, into an annex attached to the back of the paleontology hall like a caboose.”

“Tassy unlocked a small room crowded with metal cabinets. Just inside the door, partially wrapped in plastic, stood what resembled a hairy umbrella stand. This, Tassy explained, was the leg of a woolly mammoth, which had been found, frozen and desiccated, on an island off northern Siberia. When I looked at it more closely, I could see that the skin of the leg had been stitched together, like a moccasin. The hair was a very dark brown and seemed, even after more than ten thousand years, to be almost perfectly preserved.”

“Tassy opened up one of the metal cabinets and placed the contents on a wooden table. These were the teeth that Longueuil had schlepped down the Ohio River. They were huge and knobby and blackened.”

“This is the Mona Lisa of paleontology,” Tassy said, pointing to the largest of the group. “The beginning of everything. It’s incredible because Cuvier himself made the drawing of this tooth. So he looked at it very carefully.” Tassy pointed out to me the original catalog numbers, which had been painted on the teeth in the eighteenth century and were now so faded they could barely be made out.”

“I picked up the largest tooth in both hands. It was indeed a remarkable object. It was around eight inches long and four across—about the size of a brick and nearly as heavy. The cusps—four sets—were pointy, and the enamel was still largely intact. The roots, as thick as ropes, formed a solid mass the color of mahogany.”

“From an evolutionary perspective, there’s actually nothing strange about a mastodon’s molars. Mastodon teeth, like most other mammalian teeth, are composed of a core of dentin surrounded by a layer of harder but more brittle enamel. About thirty million years ago, the proboscidean line that would lead to mastodons split off from the line that would lead to mammoths and elephants. The latter would eventually evolve its more sophisticated teeth, which are made up of enamel-covered plates that have been fused into a shape a bit like a bread loaf. ”

“This arrangement is a lot tougher, and it allowed mammoths—and still allows elephants—to consume an unusually abrasive diet. Mastodons, meanwhile, retained their relatively primitive molars (as did humans) and just kept chomping away. Of course, as Tassy pointed out to me, the evolutionary perspective is precisely what Cuvier lacked, which in some ways makes his achievements that much more impressive.

“Sure, he made errors,” Tassy said. “But his technical works, most of them are splendid. He was a real fantastic anatomist.”

“After we had examined the teeth for a while longer, Tassy took me up to the paleontology hall. Just beyond the entrance, the giant femur sent to Paris by Longueuil was displayed, mounted on a pedestal. It was as wide around as a fencepost. French schoolchildren were streaming past us, yelling excitedly. Tassy had a large ring of keys, which he used to open up various drawers underneath the glass display cases. He showed me a mammoth tooth that had been examined by Cuvier and bits of various other extinct species that Cuvier had been the first to identify.”

“Then he took me to look at the Maastricht animal, still today one of the world’s most famous fossils. (Though the Netherlands has repeatedly asked for it back, the French have held on to it for more than two hundred years.) In the eighteenth century, the Maastricht fossil was thought by some to belong to a strange crocodile and by others to be from a snaggle-toothed whale. Cuvier would eventually attribute it, yet again correctly, to a marine reptile. (The creature later would be dubbed a mosasaur.)”

“Around lunchtime, I walked Tassy back to his office. Then I wandered through the gardens to the restaurant next to Cuvier’s old house. Because it seemed like the thing to do, I ordered the Menu Cuvier—your choice of entrée plus dessert. As I was working my way through the second course—a very tasty cream-filled tart—I began to feel uncomfortably full. I was reminded of a description I had read of the anatomist’s anatomy. During the Revolution, Cuvier was thin. In the years he lived on the museum grounds, he grew stouter and stouter, until, toward the end of his life, he became enormously fat.”

“WITH his lecture on “the species of elephants, both living and fossil,” Cuvier had succeeded in establishing extinction as a fact. But his most extravagant assertion—that ” “there had existed a whole lost world, filled with lost species—remained just that. If there had indeed been such a world, traces of other extinct animals ought to be findable. So Cuvier set out to find them.”

“As it happens, Paris in the seventeen-nineties was a fine place to be a paleontologist. The hills to the north of the city were riddled with quarries that were actively producing gypsum, the main ingredient of plaster of Paris. (The capital grew so haphazardly over so many mines that by Cuvier’s day cave-ins were a major hazard.) Not infrequently, miners came upon weird bones, which were prized by collectors, even though they had no real idea what they were collecting. With the help of one such enthusiast, Cuvier had soon assembled the pieces of another extinct animal, which he called l’animal moyen de Montmartre—the medium-sized animal from Montmartre.”

“All the while, Cuvier was soliciting specimens from other naturalists in other parts of Europe. Owing to the reputation the French had earned for seizing objects of value, few collectors would send along actual fossils. But detailed drawings began to arrive from, among other places, Hamburg, Stuttgart, Leiden, and Bologna. “I should say that I have been supported with the most ardent enthusiasm … by all Frenchmen and foreigners who cultivate or love the sciences,” Cuvier wrote appreciatively.”

“By 1800, which is to say four years after the elephant paper, Cuvier’s fossil zoo had expanded to include twenty-three species he deemed to be extinct. These included: a pygmy hippopotamus, whose remains he discovered in a storeroom at the Paris museum; an elk with enormous antlers whose bones had been found in Ireland; and a large bear—what now would be known as a cave bear—from Germany. The Montmartre animal had, by this point, divided, or multiplied, into six separate species. (Even today, little is known about these species, except that they were ungulates and lived some thirty million years ago.) “If so many lost species have been restored in so little time, how many must be supposed to exist still in the depths of the earth?” Cuvier asked.”

“Cuvier had a showman’s flair and, long before the museum employed public relations professionals, knew how to grab attention. (“He was a man who could have been a star on television today” is how Tassy put it to me.) At one point, the Parisian gypsum mines yielded a fossil of a rabbit-sized creature with a narrow body and a squarish head. Cuvier concluded, based on the shape of its teeth, that the fossil belonged to a marsupial. This was a bold claim, as there were no known marsupials in the Old World. To heighten the drama, Cuvier announced he would put his identification to a public test.”

“Marsupials have a distinctive pair of bones, now known as epipubic bones, that extend from their pelvis. Though these bones were not visible in the fossil as it was presented to him, Cuvier predicted that if he scratched around, the missing bones would be revealed. He invited Paris’s scientific elite to gather and watch as he picked away at the fossil with a fine needle. Voilà, the bones appeared. (A cast of the marsupial fossil is on display in Paris in the paleontology hall, but the original is deemed too valuable to be exhibited and so is kept in a special vault.)”

“Cuvier staged a similar bit of paleontological performance art during a trip to the Netherlands. In a museum in Haarlem, he examined a specimen that consisted of a large, half-moon-shaped skull attached to part of a spinal column. The three-foot-long fossil had been discovered nearly a century earlier and had been attributed—rather curiously, given the shape of the head—to a human. (It had even been assigned a scientific name: Homo diluvii testis, or “man who was witness to the Flood.”) To rebut this identification, Cuvier first got hold of an ordinary salamander skeleton. Then, with the approval of the Haarlem museum’s director, he began chipping away at the rock around the “deluge man’s” spine. ”

“When he uncovered the fossil animal’s forelimbs, they were, just as he had predicted, shaped like a salamander’s. The creature was not an antediluvian human but something far weirder: a giant amphibian.”

“The more extinct species Cuvier turned up, the more the nature of the beasts seemed to change. Cave bears, giant sloths, even giant salamanders—all these bore some relationship to species still alive. But what to make of a bizarre fossil that had been found in a limestone formation in Bavaria? Cuvier received an engraving of this fossil from one of his many correspondents. It showed a tangle of bones, including what looked to be weirdly long arms, skinny fingers, and a narrow beak. The first naturalist to examine it had speculated that its owner had been a sea animal and had used its elongated arms as paddles. Cuvier, on the basis of the engraving, determined—shockingly—that the animal was actually a flying reptile. He called it a ptero-dactyle, meaning “wing-fingered.”

Read TheSixth Extinction, pages 81-161

“CHAPTER III

THE ORIGINAL PENGUIN

Pinguinus impennis

The word “catastrophist

”

was coined in 1832 by William Whewell, one of the first presidents of the Geological Society of London, who also bequeathed to English “anode,” “cathode,” “ion,” and “scientist.” Although the term would later pick up pejorative associations, which stuck to it like burrs, this was not Whewell’s intention. When he proposed it, Whewell made it clear that he considered himself a “catastrophist,” and that most of the other scientists he knew were catastrophists too. Indeed, there was really only one person he was acquainted with ” “whom the label did not fit, and that was an up-and-coming young geologist named Charles Lyell. For Lyell, Whewell came up with yet another neologism. He called him a “uniformitarian.”

“Lyell had grown up in the south of England, in the sort of world familiar to fans of Jane Austen. He’d then attended Oxford and trained to become a barrister. Failing eyesight made it difficult for him to practice law, so he turned to the natural sciences instead. As a young man, Lyell made several trips to the Continent and became friendly with Cuvier, at whose house he dined often. He found the older man to be personally “very obliging”—Cuvier allowed him to make casts of several famous fossils to take back with him to England—but Cuvier’s vision of earth history Lyell regarded as thoroughly unpersuasive.”

“When Lyell looked (admittedly myopically) at the rock outcroppings of the British countryside or at the strata of the Paris basin or at the volcanic islands near Naples, he saw no evidence of cataclysm. In fact, quite the reverse: he thought it unscientific (or, as he put it, “unphilosophical”) to imagine that change in the world had ever occurred for different reasons or at different rates than it did in the present day. According to Lyell, every feature of the landscape was the result of very gradual processes operating over countless millennia—processes like sedimentation, erosion, and vulcanism, which were all still readily observable. For generations of geology students, Lyell’s thesis would be summed up as “The present is the key to the past.”

“As far as extinction was concerned, this, too, according to Lyell, occurred at a very slow pace—so slow that, at any given time, in any given place, it would not be surprising were it to go unnoticed. The fossil evidence, which seemed to suggest that species had at various points died out en masse, was a sign that the record was unreliable. Even the idea that the history of life had a direction to it—first reptiles, then mammals—was mistaken, another faulty inference drawn from inadequate data. All manner of organisms had existed in all eras, and those that had apparently vanished for good could, under the right circumstances, pop up again. Thus “the huge iguanodon might reappear in the woods, and the ichthyosaur in the sea, while the pterodactyle might flit again through umbrageous groves of tree-ferns.” It is clear, Lyell wrote, “that there is no foundation in geological facts for the popular theory of the successive development of the animal and vegetable world.”

“Lyell published his ideas in three thick volumes, Principles of Geology: Being an Attempt to Explain the Former Changes of the Earth’s Surface by Reference to Causes Now in Operation. The work was aimed at a general audience, which embraced it enthusiastically. A first print run of forty-five hundred copies quickly sold out, and a second run of nine thousand was ordered up. (In a letter to his fiancée, Lyell boasted that this represented “at least 10 times” as many books as any other English geologist had ever sold.) Lyell became something of a celebrity—the Steven Pinker of his generation—and when he spoke in Boston more than four thousand people tried to get tickets.”

“for the sake of clarity (and a good read), Lyell had caricatured his opponents, making them sound a great deal more “unphilosophical” than they actually were. They returned the favor. A British geologist named Henry De la Beche, who had a knack for drawing, poked fun at Lyell’s ideas about eternal return. He produced a cartoon showing Lyell in the form of a nearsighted ichthyosaur, pointing to a human skull and lecturing to a group of giant reptiles.”

“You will at once perceive,” Professor Ichthyosaurus tells his pupils in the caption, “that the skull before us belonged to some of the lower order of animals; the teeth are very insignificant, the power of the jaws trifling, and altogether it seems wonderful how the creature could have procured food.” De la Beche called the sketch “Awful Changes.”

“AMONG the readers who snapped up the Principles was Charles Darwin. Twenty-two years old and fresh out of Cambridge, Darwin had been invited to serve as a sort of gentleman’s companion to the captain of the HMS Beagle, Robert FitzRoy. The ship was headed to South America to survey the coast and resolve various mapping discrepancies that hindered navigation. (The Admiralty was particularly interested in finding the best approach to the Falkland Islands, which the British had recently assumed control of.) The voyage, which would last until Darwin was twenty-seven, would take him from Plymouth to Montevideo, through the Strait of Magellan, up to the Galápagos Islands, across the South Pacific to Tahiti, on to New Zealand, Australia, and Tasmania, across the Indian Ocean to Mauritius, around the Cape of Good Hope, and back again to South America. In the popular imagination, the journey is usually seen as the time when Darwin, encountering a wild assortment of giant tortoises, seafaring lizards, and finches with beaks of every imaginable shape and size, discovered natural selection. In fact, Darwin developed his theory only after his return to England, when other naturalists sorted out the jumble of specimens he had shipped back.

“It would be more accurate to describe the voyage of the Beagle as the period when Darwin discovered Lyell. Shortly before the ship’s departure, FitzRoy presented Darwin with a copy of volume one of the Principles. Although he was horribly seasick on the first leg of the journey (as he was on many subsequent legs), Darwin reported that he read Lyell “attentively” as the ship headed south. The Beagle made its first stop at St. Jago—now Santiago—in the Cape Verde Islands, and Darwin, eager to put his new knowledge to work, spent several days collecting specimens from its rocky cliffs. One of Lyell’s central claims was that some areas of the earth were gradually rising, just as others were gradually subsiding. (Lyell further contended that these phenomena were always in balance, so as to “preserve the uniformity of the general relations of the land and sea.”) St. Jago seemed to prove his point. “ The island was clearly volcanic in origin, but it had several curious features, including a ribbon of white limestone halfway up the dark cliffs. The only way to explain these features, Darwin concluded, was as evidence of uplift. The very first place “which I geologised convinced me of the infinite superiority of Lyell’s views,” he would later write. So taken was Darwin with volume one of the Principles that he had volume two shipped to him for pickup at Montevideo. Volume three, it seems, caught up with him in the Falklands.”

“While the Beagle was sailing along the west coast of South America, Darwin spent several months exploring Chile. He was resting after a hike one afternoon near the town of Valdivia when the ground beneath him began to wobble, as if made of jelly. “One second of time conveys to the mind a strange idea of insecurity, which hours of reflection could never create,” he wrote.” “Several days after the earthquake, arriving in Concepción, Darwin found the entire city had been reduced to rubble. “It is absolutely true, there is not one house left habitable,” he reported. The scene was the “most awful yet interesting spectacle” he’d ever witnessed. A series of surveying measurements that FitzRoy took around Concepción’s harbor showed that the quake had elevated the beach by nearly eight feet. Once again, Lyell’s Principles appeared to be rather spectacularly confirmed. Given enough time, Lyell argued, repeated quakes could raise an entire mountain chain many thousands of feet high.”

“he more Darwin explored the world, the more Lyellian it seemed to him to be. Outside the port of Valparaiso, he found deposits of marine shells far above sea level. These he took to be the result of many episodes of elevation like the one he’d just witnessed. “I have always thought that the great merit of the Principles was that it altered the whole tone of one’s mind,” he would later write. (While in Chile, Darwin also discovered a new and rather remarkable species of frog, which became known as the Chile Darwin’s frog. Males of the species incubated their tadpoles in their vocal sacs. Recent searches have failed to turn up any Chile Darwin’s frogs, and the species is now believed to be extinct.)”

“Toward the end of the Beagle’s voyage, Darwin encountered coral reefs. These provided him with his first major breakthrough, a startling idea that would ease his entrée into London’s scientific circles. Darwin saw that the key to understanding coral reefs was the interplay between biology and geology. If a reef formed around an island or along a continental margin that was slowly sinking, the corals, by growing slowly upward, could maintain their position relative to the water. Gradually, as the land subsided, the corals would form a barrier reef. If, eventually, the land sank away entirely, the reef would form an atoll.”

“Darwin’s account went beyond and to a certain extent contradicted Lyell’s; the older man had hypothesized that reefs grew from the rims of submerged volcanoes. But Darwin’s ideas were so fundamentally Lyellian in nature that when, upon his return to England, Darwin presented them to Lyell, the latter was delighted. As the historian of science Martin Rudwick has put it, Lyell “recognized that Darwin had out-Lyelled him.”

“One biographer summed up Lyell’s influence on Darwin as follows: “Without Lyell there would have been no Darwin.” Darwin himself, after publishing his account of the voyage of the Beagle and also a volume on coral reefs, wrote, “I always feel as if my books came half out of Lyell’s brains.”

“LYELL, who saw change occurring always and everywhere in the world around him, drew the line at life. That a species of plant or animal might, over time, give rise to a new one he found unthinkable, and he devoted much of the second volume of the Principles to attacking the idea, at one point citing Cuvier’s mummified cat experiment in support of his objections.”

“Lyell’s adamant opposition to transmutation, as it was known in London, is almost as puzzling as Cuvier’s. New species, Lyell realized, regularly appeared in the fossil record. But how they originated was an issue he never really addressed, except to say that probably each one had begun with “a single pair, or individual, where an individual was sufficient” and multiplied and spread out from there. This process, which seemed to depend on divine or at least occult intervention, was clearly at odds with the precepts he had laid out for geology. Indeed, as one commentator observed, it seemed to require “exactly the kind of miracle” that Lyell had rejected.”

“With his theory of natural selection, Darwin once again “out-Lyelled” Lyell. Darwin recognized that just as the features of the inorganic world—deltas, river valleys, mountain chains—were brought into being by gradual change, the organic world similarly was subject to constant flux. Ichthyosaurs and plesiosaurs, birds and fish and—most discomfiting of all—humans had come into being through a process of transformation that took place over countless generations. This process, though imperceptibly slow, was, according to Darwin, still very much going on; in biology, as in geology, the present was the key to the past. In one of the most often-quoted passages of On the Origin of Species, Darwin wrote:

It may be said that natural selection is daily and hourly scrutinising, throughout the world, every variation, even the slightest; rejecting that which is bad, preserving and adding up all that is good; silently and insensibly working, whenever and wherever opportunity offers.”

“Natural selection eliminated the need for any sort of creative miracles. Given enough time for “every variation, even the slightest” to accumulate, new species would emerge from the old. Lyell this time was not so quick to applaud his protégé’s work. He only grudgingly accepted Darwin’s theory of “descent with modification,” so grudgingly that his stance seems to have eventually ruined their friendship.”

“Darwin’s theory about how species originated doubled as a theory of how they vanished. Extinction and evolution were to each other the warp and weft of life’s fabric, or, if you prefer, two sides of the same coin. “The appearance of new forms and the disappearance of old forms” were, Darwin wrote, “bound together.” Driving both was the “struggle for existence,” which rewarded the fit and eliminated the less so.”

“The theory of natural selection is grounded on the belief that each new variety, and ultimately each new species, is produced and maintained by having some advantage over those with which it comes into competition; and the consequent extinction of less favoured forms almost inevitably follows.

”

“Darwin used the analogy of domestic cattle. When a more vigorous or productive variety was introduced, it quickly supplanted other breeds. In Yorkshire, for example, he pointed out, “it is historically known that the ancient black cattle were displaced by the long-horns,” and that these were subsequently “swept away” by the short-horns, “as if by some murderous pestilence.”

Darwin stressed the simplicity of his account. Natural selection was such a powerful force that none other was needed. Along with miraculous origins, world-altering catastrophes could be dispensed with. “The whole subject of the extinction of species has been involved in the most gratuitous mystery,” he wrote, implicitly mocking Cuvier.”

“From Darwin’s premises, an important prediction followed. If extinction was driven by natural selection and only by natural selection, the two processes had to proceed at roughly the same rate. If anything, extinction had to occur more gradually.

“The complete extinction of the species of a group is generally a slower process than their production,” he observed at one point.”

“No one had ever seen a new species produced, nor, according to Darwin, should they expect to. Speciation was so drawn out as to be, for all intents and purposes, unobservable. “We see nothing of these slow changes in progress,” he wrote. It stood to reason that extinction should have been that much more difficult to witness. And yet it wasn’t. In fact, during the years Darwin spent holed up at Down House, developing his ideas about evolution, the very last individuals of one of Europe’s most celebrated species, the great auk, disappeared. What’s more, the event was painstakingly chronicled by British ornithologists. Here Darwin’s theory was directly contradicted by the facts, with potentially profound implications.”

“THE Icelandic Institute of Natural History occupies a new building on a lonely hillside outside Reykjavik. The building has a tilted roof and tilted glass walls and looks a bit like the prow of a ship. It was designed as a research facility, with no public access, which means that a special appointment is needed to see any of the specimens in the institute’s collection. These specimens, as I learned on the day of my own appointment, include: a stuffed tiger, a stuffed kangaroo, and a cabinet full of stuffed birds of paradise.”

“The reason I’d arranged to visit the institute was to see its great auk. Iceland enjoys the dubious distinction of being the bird’s last known home, and the specimen I’d come to look at was killed somewhere in the country—no one is sure of the exact spot—in the summer of 1821. The bird’s carcass was purchased by a Danish count, Frederik Christian Raben, who had come to Iceland expressly to acquire an auk for his collection (and had nearly drowned in the attempt). “Raben took the specimen home to his castle, and it remained in private hands until 1971, when it came up for auction in London. The Institute of Natural History solicited donations, and within three days Icelanders contributed the equivalent of ten thousand British pounds to buy the auk back. (One woman I spoke to, who was ten years old at the time, recalled emptying her piggy bank for the effort.) Icelandair provided two free seats for the homecoming, one for the institute’s director and the other for the boxed bird.”

“Guðmundur Guðmundsson, who’s now the institute’s deputy director, had been assigned the task of showing me the auk. Guðmundsson is an expert on foraminifera, tiny marine creatures that form intricately shaped shells, known as “tests.” On our way to see the bird, we stopped at his office, which was filled with boxes of little glass tubes, each containing a sampling of tests that rattled like sprinkles when I picked it up. Guðmundsson told me that in his spare time he did translating. A few years ago he had completed the first Icelandic rendering of On the Origin of Species. He’d found Darwin’s prose quite difficult—“sentences inside sentences inside sentences”—and the book, Uppruni Tegundanna, had not sold well, perhaps because so many Icelanders are fluent in English.

We made our way to the storeroom for the institute’s collection. The stuffed tiger, wrapped in plastic, looked ready to lunge at the stuffed kangaroo. The great auk—Pinguinus impennis—was standing off by itself, in a specially made Plexiglas case. It was perched on a fake rock, next to a fake egg.”

“As the name suggests, the great auk was a large bird; adults grew to be more than two and a half feet tall. The auk could not fly—it was one of the few flightless birds of the Northern Hemisphere—and its stubby wings were almost comically undersized for its body. The auk in the case had brown feathers on its back; probably these were black when the bird was alive but had since faded. “UV light,” Guðmundsson said gloomily. “It destroys the plumage.” The auk’s chest feathers were white, and there was a white spot just beneath each eye. The bird had been stuffed with its most distinctive feature—its large, intricately grooved beak—tipped slightly into the air. This lent it a look of mournful hauteur.

”

“Guðmundsson explained that the great auk had been on display in Reykjavik until 2008, when the institute was restructured by the Icelandic government. At that point, another agency was supposed to create a new home for the bird, but various mishaps, including Iceland’s financial crisis, had prevented this from happening, which is why Count Raben’s auk was sitting on its fake rock in the corner of the storeroom. On the rock, there was a painted inscription, which Guðmundsson translated for me: THE BIRD WHO IS HERE FOR SHOW WAS KILLED IN 1821. IT IS ONE OF THE FEW GREAT AUKS THAT STILL EXIST.”

“N its heyday, which is to say, before humans figured out how to reach its nesting grounds, the great auk ranged from Norway over to Newfoundland and from Italy to Florida, and its population probably numbered in the millions. When the first settlers arrived in Iceland from Scandinavia, great auks were so common that they were regularly eaten for dinner, and their remains have been found in the tenth-century equivalent of household trash. While I was in Reykjavik, I visited a museum built over the ruins of what’s believed to be one of the most ancient structures in Iceland—a longhouse constructed out of strips of turf. “According to one of the museum’s displays, the great auk was “easy prey” for Iceland’s medieval inhabitants. In addition to a pair of auk bones, the display featured a video re-creation of an early encounter between man and bird. In the video, a shadowy figure crept along a rocky shore toward a shadowy auk. When he drew close enough, the figure pulled out a stick and clubbed the animal over the head. The auk responded with a cry somewhere between a honk and a grunt. I found the video grimly fascinating “cinating and watched it play through a half a dozen times. Creep, clobber, squawk. Repeat.”

“As best as can be determined, great auks lived much as penguins do. In fact, great auks were the original “penguins.” They were called this—the etymology of “penguin” is obscure and may or may not be traced to the Latin pinguis, meaning “fat”—by European sailors who encountered them in the North Atlantic. Later, when subsequent generations of sailors met similar-colored flightless birds in the Southern Hemisphere, they used the same name, which led to much confusion, since auks and penguins belong to entirely different families. (Penguins constitute their own family, while auks are members of the family that includes puffins and guillemots; genetic analysis has shown that razorbills are the great auk’s closest living relatives.)”

“Like penguins, great auks were fantastic swimmers—eyewitness accounts attest to the birds’ “astonishing velocity” in the water—and they spent most of their lives at sea. But during breeding season, in May and June, they waddled ashore in huge numbers, and here lay their vulnerability. Native Americans clearly hunted the great auk—one ancient grave in Canada was found to contain more than a hundred great auk beaks—as did paleolithic Europeans: great auk bones have been found at archaeological sites in, among other places, Denmark, Sweden, Spain, Italy, and Gibraltar. By the time the first settlers got to Iceland, many of its breeding sites had already been plundered and its range was probably much reduced. Then came the wholesale slaughter.”

“Lured by the rich cod fishery, Europeans began making regular voyages to Newfoundland in the early sixteenth century. Along the way, they encountered a slab of pinkish granite about fifty acres in area, which rose just above the waves. In the spring, the slab was covered with birds, standing, in a manner of speaking, shoulder to shoulder. Many of these were gannets and guillemots; the rest were great auks. The slab, about forty miles off Newfoundland’s northeast coast, became known as the Isle of Birds or, in some accounts, Penguin Island; today it is known as Funk Island. Toward the end of a long transatlantic journey, when provisions were running low, fresh meat was prized, and the ease with which auks could be picked off the slab was soon noted. In an account from 1534, the French explorer Jacques Cartier wrote that some of the Isle of Birds’ inhabitants were “as large as geese.”

“They are always in the water, not being able to fly in the air, inasmuch as they have only small wings … with which … they move as quickly along the water as the other birds fly through the air. And these birds are so fat it is marvellous. In less than half an hour we filled two boats full of them, as if they had been stones, so that besides them which we did not eat fresh, every ship did powder and salt five or six barrels full of them.

”

“A British expedition that landed on the island a few years later found it “full of great foules.” The men drove a “great number of the foules” into their ships and pronounced the results to be quite tasty—“very good and nourishing meat.” A 1622 account by a captain named Richard Whitbourne describes great auks being driven onto boats “by hundreds at a time as if God had made the innocency of so poor a creature to become such an admirable instrument for the sustenation of Man.”

“Over the next several decades, other uses for the great auk were found besides “sustenation.” (As one chronicler observed, “the great auks of Funk Island were exploited in every way that human ingenuity could devise.”) Auks were used as fish bait, as a source of feathers for stuffing mattresses, and as fuel. Stone pens were erected on Funk Island—vestiges of these are still visible today—and the birds were herded into the enclosures until someone could find time to butcher them. Or not. According to an English seaman named Aaron Thomas, who sailed to Newfoundland on the HMS Boston:

“If you come for their Feathers you do not give yourself the trouble of killing them, but lay hold of one and pluck the best of the Feathers. You then turn the poor Penguin adrift, with his skin half naked and torn off, to perish at his leisure.”

“There are no trees on Funk Island, and hence nothing to burn. This led to another practice chronicled by Thomas.”

“You take a kettle with you into which you put a Penguin or two, you kindle a fire under it, and this fire is absolutely made of the unfortunate Penguins themselves. Their bodys being oily soon produce a Flame.”

“It’s been estimated that when Europeans first landed at Funk Island, they found as many as a hundred thousand pairs of great auks tending to a hundred thousand eggs. (Probably great auks produced only one egg a year; these were about five inches long and speckled, Jackson Pollock–like, in brown and black.) Certainly the island’s breeding colony must have been a large one to persist through more than two centuries of depredation. By the late seventeen hundreds, though, the birds’ numbers were in sharp decline. The feather trade had become so lucrative that teams of men were spending the entire summer on Funk, scalding and plucking. In 1785, George Cartwright, an English trader and explorer, observed of these teams: “The destruction which they have made is incredible.” If a stop were not soon put to their efforts, he predicted, the great auk would soon “be diminished to almost nothing.”

“Whether the teams actually managed to kill off every last one of the island’s auks or whether the slaughter simply reduced the colony to the point that it became vulnerable to other forces is unclear. (Diminishing population density may have made survival less likely for the remaining individuals, a phenomenon that’s known as the Allee effect.) In any event, the date that’s usually given for the extirpation of the great auk from North America is 1800. Some thirty years later, while working on The Birds of America, John James Audubon traveled to Newfoundland in search of great auks to paint from life. He couldn’t find any, and for his illustration had to make do with a stuffed bird from Iceland that had been acquired by a dealer in London. In his description of the great auk, Audubon wrote that it was “rare and accidental on the banks of Newfoundland” and that it was “said to breed on a rock on that island,” a curious contradiction since no breeding bird can be said to be “accidental.”

“ONCE the Funk Island birds had been salted, plucked, and deep-fried into oblivion, there was only one sizable colony of great auks left in the world, on an island called the Geirfuglasker, or great auk skerry, which lay about thirty miles off southwestern Iceland’s Reykjanes Peninsula. Much to the auk’s misfortune, a volcanic eruption destroyed the Geirfuglasker in 1830. This left the birds one solitary refuge, a speck of an island known as Eldey. By this point, the great auk was facing a new threat: its own rarity. Skins and eggs were avidly sought by gentlemen, like Count Raben, who wanted to fill out their collections. It was in the service of such enthusiasts that the very last known pair of auks was killed on Eldey in 1844.”

“Before setting out for Iceland, I’d decided that I wanted to see the site of the auk’s last stand. Eldey is only about ten miles off the Reykjanes Peninsula, which is just south of Reykjavik. But getting out to the island proved to be way more difficult to arrange than I’d imagined. Everyone I contacted in Iceland told me that no one ever went there. Eventually, a friend of mine “who’s from Iceland got in touch with his father, who’s a minister in Reykjavik, who contacted a friend of his, who runs a nature center in a tiny town on the peninsula called Sandgerði. The head of the nature center, Reynir Sveinsson, in turn, found a fisherman, Halldór Ármannsson, who said he’d be willing to take me, but only if the weather was fair; if it was rainy or windy, the trip would be too dangerous and nausea-inducing, and he wouldn’t want to risk it.”

“Fortunately, the weather on the day we’d fixed turned out to be splendid. I met Sveinsson at the nature center, which features an exhibit on a French explorer, Jean-Baptiste Charcot, who died when his ship, the infelicitously named Pourquoi-Pas, sunk off Sandgerði in 1936. We walked over to the harbor and found Ármannsson loading a chest onto his boat, the Stella. He explained that inside the chest was an extra life raft. “Regulations,” he shrugged. Ármannsson had also brought along his fishing partner and a cooler filled with soda and cookies. He seemed pleased to be making a trip that didn’t involve cod.”

“We motored out of the harbor and headed south, around the Reykjanes Peninsula. It was clear enough that we could see the snow-covered peak of Snæfellsjökull, more than sixty miles away. (To English speakers, Snæfellsjökull is probably best known as the spot where in Jules Verne’s A Journey to the Center of the Earth the hero finds a tunnel through the globe.) Eldey, being much shorter than Snæfellsjökull, was not yet visible. Sveinsson explained that Eldey’s name means “fire island.” He said that although he’d spent his entire life in the area, he’d never before been out to it. He’d brought along a fancy camera and was shooting pictures more or less continuously.”

“As Sveinnson snapped away, I chatted with Ármannsson inside the Stella’s small cabin. I was intrigued to see that he had dramatically different colored eyes, one blue and one hazel. Usually, he told me, he fished for cod using a long line that extended six miles and trailed twelve thousand hooks. The baiting of the hooks was his father’s job, and it took nearly two days. A good catch could weigh more than seven metric tons. Often Ármannsson slept on the Stella, which was equipped with a microwave and two skinny berths.”

“After a while, Eldey appeared on the horizon. The island looked like the base of an enormous column, or like a giant pedestal waiting for an even more gigantic statue. When we got within maybe a mile, I could see that the top of the island, which from a distance appeared flat, was actually tilted at about a ten-degree angle. We were approaching from the shorter end, so we could look across the entire surface. It was white and appeared to be rippling. As we got closer, I realized that the ripples were birds—so many that they seemed to blanket the island—and when we got even closer, I could see that the birds were gannets—elegant creatures with long necks, cream-colored heads, and tapered beaks. Sveinsson explained that Eldey was “home to one of the world’s largest colonies of northern gannets—some thirty thousand pairs. He pointed out a pyramid-like structure atop the island. This was a platform for a webcam that Iceland’s environmental agency had set up. It was supposed to stream a live feed of the gannets to bird-watchers, but it had not functioned as planned.”

“The birds do not like this camera,” Sveinsson said. “So they fly over it and shit on it.” The guano from thirty thousand gannet pairs has given the island what looks like a coating of vanilla frosting.”

“Because of the gannets, and perhaps also because of the island’s history, visitors are not allowed to step onto Eldey without special (and hard-to-obtain) permits. When I first learned this, I was disappointed, but when we got right up to the island and I saw the way the sea beat against the cliffs, I felt relieved.”

“THE last people to see great auks alive were around a dozen Icelanders who made the trip to Eldey by rowboat. They set out one evening in June 1844, rowed through the night, and reached the island the following morning. With some difficulty, three of the men managed to clamber ashore at the only possible landing spot: a shallow shelf of rock that extends from the island to the northeast. (A fourth man who was supposed to go with them refused to on the grounds that it was too dangerous.) By this point the island’s total auk population, probably never very numerous, appears to have consisted of a single pair of birds and one egg. On catching sight of the humans, the birds tried to run, but they were too slow. Within minutes, the Icelanders had captured the auks and strangled them. The egg, they saw, had been cracked, presumably in the course of the chase, so they left it behind. Two of the men were able to jump back into the boat; the third had to be hauled through the waves with a rope.”

“The details of the great auks’ last moments, including the names of the men who killed the birds—Sigurður Iselfsson, ”

“Ketil Ketilsson, and Jón Brandsson—are known because fourteen years later, in the summer of 1858, two British naturalists traveled to Iceland in search of auks. The older of these, John Wolley, was a doctor and an avid egg collector; the younger, Alfred Newton, was a fellow at Cambridge and soon to be the university’s first professor of zoology. The pair spent several weeks on the Reykjanes Peninsula, not far from the site of what is now Iceland’s international airport, and during that time, they seem to have talked to just about everyone who had ever seen an auk, or even just heard about one, including several of the men who’d made the 1844 expedition. “he pair spent several weeks on the Reykjanes Peninsula, not far from the site of what is now Iceland’s international airport, and during that time, they seem to have talked to just about everyone who had ever seen an auk, or even just heard about one, including several of the men who’d made the 1844 expedition. The pair of birds that had been killed in that outing, they discovered, had been sold to a dealer for the equivalent of about nine pounds. The birds’ innards had been sent to the Royal Museum in Copenhagen; no one could say what had happened to the skins. (Subsequent detective work has traced the skin of the female to an auk now on display at the Natural History Museum of Los Angeles.)”

“Wolley and Newton hoped to get out to Eldey themselves. Wretched weather prevented them. “Boats and men were engaged, and stores laid in, but not a single opportunity occurred when a landing would have been practicable,” Newton would later write. “It was with heavy hearts that we witnessed the season wearing away.”

“Wolley died shortly after the pair returned to England. For Newton, the experience of the trip would prove to be life-altering. He concluded that the auk was gone—“for all practical purposes therefore we may speak of it as a thing of the past”—and he developed what one biographer referred to as a “peculiar attraction” to “extinct and disappearing faunas.” Newton realized that the birds that bred along Britain’s long coast were also in danger; he noted that they were being gunned down for sport in great numbers.”

“The bird that is shot is a parent,” he observed in an address to the British Association for the Advancement of Science. “We take advantage of its most sacred instincts to waylay it, and in depriving the parent of life, we doom the helpless offspring to “the most miserable of deaths, that by hunger. If this is not cruelty, what is?” Newton argued for a ban on hunting during breeding season, and his lobbying resulted in one of the first laws aimed at what today would be called wildlife protection: the Act for the Preservation of Sea Birds.”

“AS it happens, Darwin’s first paper on natural selection appeared in print just as Newton was returning home from Iceland. The paper, in the Journal of the Proceedings of the Linnean Society, had—with Lyell’s help—been published in a rush soon after Darwin had learned that a young naturalist named Alfred Russel Wallace was onto a similar idea. (A paper by Wallace appeared in the same issue of the Journal.) Newton read Darwin’s essay very soon after it came out, staying up late into the night to finish it, and he immediately became a convert. “It came to me like the direct revelation of a higher power,” he later recalled, “and I awoke next morning with the consciousness that there was an end of all the mystery in the simple phrase, ‘Natural Selection.’” He had, he wrote to a friend, developed a case of “pure and unmitigated Darwinism. “A few years later, Newton and Darwin became correspondents—at one point Newton sent Darwin a diseased partridge’s foot that he thought might be of interest to him—and eventually the two men paid social calls on each other.”

“Whether the subject of the great auk ever came up in their conversations is unknown. It is not mentioned in Newton and Darwin’s surviving correspondence, nor does Darwin allude to the bird or its recent demise in any of his other writings. But Darwin had to be aware of human-caused extinction. In the Galápagos, he had personally witnessed, if not exactly a case of extinction in action, then something very close to it.”

“Darwin’s visit to the archipelago took place in the fall of 1835, nearly four years into the voyage of the Beagle. On Charles Island—now Floreana—he met an Englishman named Nicholas Lawson, who was the Galápagos’s acting governor as well as the warden of a small, rather miserable penal colony. Lawson was full of useful information. Among the facts he related to Darwin was that on each of the islands in the Galápagos the tortoises had different-shaped shells. On this basis, Lawson claimed that he could “pronounce from which island any tortoise may have been brought.” Lawson also told Darwin that the tortoises’ days were numbered. “ The islands were frequently visited by whaling ships, which carried the huge beasts off as portable provisions. Just a few years earlier, a frigate visiting Charles Island had left with two hundred tortoises stowed in its hold. As a result, Darwin noted in his diary, “the numbers have been much reduced.” By the time of the Beagle’s visit, tortoises had become so scarce on Charles Island that Darwin, it seems, did not see a single one. Lawson predicted that Charles’s tortoise, known today by “the scientific name Chelonoidis elephantopus, would be entirely gone within twenty years. In fact, it probably disappeared in fewer than ten. (Whether Chelonoidis elephantopus was a distinct species or a subspecies is still a matter of debate.)”

“Darwin’s familiarity with human-caused extinction is also clear from On the Origin of Species. In one of the many passages in which he heaps scorn on the catastrophists, he observes that animals inevitably become rare before they become extinct: “we know this has been the progress of events with those animals which have been exterminated, either locally or wholly, through man’s agency.” It’s a brief allusion and, in its brevity, suggestive. Darwin assumes that his readers are familiar with such “events” and already habituated to them. He himself seems to find nothing remarkable or troubling about this. But human-caused extinction is of course troubling for many reasons, some of which have to do with Darwin’s own theory, and it’s puzzling that a writer as shrewd and self-critical as Darwin shouldn’t have noticed this.”

“n the Origin, Darwin drew no distinction between man and other organisms. As he and many of his contemporaries recognized, this equivalence was the most radical aspect of his work. Humans, just like any other species, were descended, with modification, from more ancient forebears. Even those qualities that seemed to set people apart—language, wisdom, a sense of right and wrong—had evolved in the same manner as other adaptive traits, such as longer beaks or sharper incisors. At the heart of Darwin’s theory, as one of his biographers has put it, is “the denial of humanity’s special status.”

“And what was true of evolution should also hold for extinction, since according to Darwin, the latter was merely a side effect of the former. Species were annihilated, just as they were created, by “slow-acting and still existing causes,” which is to say, through competition and natural selection; to invoke any other mechanism was nothing more than mystification. But how, then, to make sense of cases like the great auk or the Charles Island tortoise or, to continue the list, the dodo or the Steller’s sea cow? These animals had obviously not been done in by a rival species gradually evolving some competitive advantage. They had all been killed off by the same species, and all quite suddenly—in the case of the great auk and the Charles Island tortoise over the course of Darwin’s own lifetime. Either there had to be a separate category for human-caused extinction, in which case people really did deserve their “special status” as a creature outside of nature, or space in the natural order had to be made for cataclysm, in which case, Cuvier—distressingly—was right.”

“CHAPTER IV

THE LUCK OF THE AMMONITES

Discoscaphites jerseyensis”

“The hill town of Gubbio, about a hundred miles north of Rome, might be described as a municipal fossil. Its streets are so narrow that on many of them not even the tiniest Fiat has room to maneuver, and its gray stone piazzas look much as they did in Dante’s era. (In fact, it was a powerful Gubbian, installed as lord mayor of Florence, who engineered Dante’s exile, in 1302.) If you visit in winter, as I did, when the tourists are gone, the hotels shuttered, and the town’s picture-book palace deserted, it almost seems as if Gubbio has fallen under a spell and is waiting to be awoken.”

“Just beyond the edge of town a narrow gorge leads off to the northeast. The walls of the gorge, which is known as the Gola del Bottaccione, consist of bands of limestone that run in diagonal stripes. Long before people settled the region—long before people existed—Gubbio lay at the bottom of a clear, blue sea. The remains of tiny marine creatures rained down on the floor of that sea, building up year after year, century after century, millennium after millennium. In the uplift that created the Apennine Mountains, the limestone was elevated and tilted at a forty-five-degree angle. To walk up the gorge today is thus to travel, layer by layer, through time. In the space of a few hundred yards, you can cover almost a hundred million years.”

“The Gola del Bottaccione is now a tourist destination in its own right, though for a more specialized crowd. It is here that in the late nineteen-seventies, a geologist named Walter Alvarez, who had come to study the origins of the Apennines, ended up, more or less by accident, rewriting the history of life. In the gorge, he discovered the first traces of the giant asteroid that ended the Cretaceous period and caused what may have been the worst day ever on planet earth. By the time the dust—in this case, literal as much as figurative—had settled, some three-quarters of all species had been wiped out.

“The evidence of the asteroid’s impact lies in a thin layer of clay about halfway up the gorge. Sightseers can park at a turnoff constructed nearby. There’s also a little kiosk explaining, in Italian, the site’s significance. The clay layer is easy to spot. It’s been gouged out by hundreds of fingers, a bit like the toes of the bronze St. Peter in Rome, worn down by the kisses of pilgrims. The day I visited was gray and blustery, and I had the place to myself. I wondered what had prompted all that fingering. Was it simple curiosity? A form of geologic rubbernecking? Or was it something more empathetic: the desire to make contact—however attenuated—with a lost world? I, too, of course, had to stick my finger in. I poked around in the groove and scraped out a pebble-sized piece of clay. It was the color of worn brick and the consistency of dried mud. I put it in an old candy wrapper and stuck it in my pocket—my own little chunk of planetary disaster.”

“WALTER Alvarez came from a long line of distinguished scientists. His great-grandfather and grandfather were both noted physicians, and his father, Luis, was a physicist at the University of California-Berkeley. But it was his mother who took him for long walks in the Berkeley hills and got him interested in geology. Walter attended graduate school at Princeton, then went to work for the oil industry. (He was living in Libya when Muammar Gaddafi took over the country in 1969.) A few years later he got a research post at the Lamont-Doherty Earth Observatory, across the Hudson from Manhattan. At the time, what’s sometimes called the “plate tectonics revolution” was sweeping through the profession, and just about everyone at Lamont got swept up in it.”

“Alvarez decided to try to figure out how, on the basis of plate tectonics, the Italian peninsula had come into being. Key to the project was a kind of reddish limestone, known as the scaglia “rosso, which can be found, among other places, in the Gola del Bottaccione. The project moved forward, got stuck, and shifted direction. “In science, sometimes it’s better to be lucky than smart,” he would later say of these events. Eventually, he found himself working in Gubbio with an Italian geologist named Isabella Premoli Silva, who was an expert on foraminifera.”

“Foraminifera, or “forams” for short, are the tiny marine creatures that create little calcite shells, or tests, which drift down to the ocean floor once the animal inside has died. The tests have a distinctive shape, which varies from species to species; some look (under magnification) like beehives, others like braids or bubbles or clusters of grapes. Forams tend to be widely distributed and abundantly preserved, and this makes them extremely useful as index fossils: on the basis of which species of forams are found in a given layer of rock, an expert like Silva can tell the rock’s age. As they worked their way up the Gola del Bottaccione, Silva pointed out to Alvarez a curious sequence. The limestone from the last stage of the Cretaceous period contained diverse, abundant, and relatively large forams, many as big as grains of sand. Directly above that, there was a layer of clay about half an inch thick with no forams in it. Above the clay there was limestone with more forams, but these belonged to only a handful of species, all of them very tiny and all totally different from the larger ones below.”

“Alvarez had been schooled in, to use his phrase, a “kind of hard-core uniformitarianism.” He’d been trained to believe, after Lyell and Darwin, that the disappearance of any group of organisms had to be a gradual process, with one species slowly dying out, then another, then a third, and so on. Looking at the sequence in the Gubbio limestone, though, he saw something different. The many species of forams in the lower layer seemed to disappear suddenly and all more or less at the same time; the whole process, Alvarez would later recall, certainly “looked very abrupt.” Then there was the odd matter of timing. The king-sized forams appeared to vanish right around the point the last of the dinosaurs were known to have died off. This struck Alvarez as more than just a coincidence. He thought it would be interesting to know exactly how much time that half-inch of clay represented.”

“In 1977, Alvarez got a job at Berkeley, where his father, Luis, was still working, and he brought with him to California his samples from Gubbio. While Walter had been studying plate tectonics, Luis had won a Nobel Prize. He’d also developed the first linear proton accelerator, invented a new kind of bubble chamber, designed several innovative radar systems, and codiscovered tritium. Around Berkeley, Luis had become known as the “wild idea man.” Intrigued by a debate over whether there were treasure-filled chambers inside Egypt’s second-largest pyramid, he’d at one point designed a test that required installing a muon detector in the desert. (The detector showed that the pyramid was, in fact, solid rock.) At another point, he’d become interested in the Kennedy assassination and had performed an experiment that involved wrapping cantaloupes in shipping tape and shooting them with a rifle. (The experiment demonstrated that the movement of the president’s head after he was hit was consistent with the findings of the Warren Commission.) “When Walter told his father about the puzzle from Gubbio, Luis was fascinated. It was Luis who came up with the wild idea of clocking the clay using the element iridium.”

“Iridium is extremely rare on the surface of the earth but much more common in meteorites. In the form of microscopic grains of cosmic dust, bits of meteorites are constantly raining down on the planet. Luis reasoned that the longer it had taken the clay layer to accumulate, the more cosmic dust would have fallen; thus the more iridium it would contain. He contacted a Berkeley colleague, Frank Asaro, whose lab was one of the few with the right kind of equipment for this sort of analysis. Asaro agreed to run tests on a dozen samples, though he said he very much doubted anything would come of it. Walter gave him some limestone from above the clay layer, some from below it, and some of the clay itself. Then he waited. Nine months later, he got a call. There was something seriously wrong with the samples from the clay layer. The amount of iridium in them was off the charts.”

“No one knew what to make of this. Was it a weird anomaly, or something more significant? Walter flew to Denmark, to collect some late-Cretaceous sediments from a set of limestone cliffs known as Stevns Klint. At Stevns Klint, the end of the Cretaceous period shows up as a layer of clay that’s jet black and smells like dead fish. When the stinky Danish samples were analyzed, they, too, revealed astronomical levels of iridium. A third set of samples, from the South Island of New Zealand, also showed an iridium “spike” right at the end of the Cretaceous.”

“Luis, according to a colleague, reacted to the news “like a shark smelling blood”; he sensed the opportunity for a great discovery. The Alvarezes batted around theories. But all the ones they could think of either didn’t fit the available data or were ruled out by further tests. Then, finally, after almost a year’s worth of dead ends, they arrived at the impact hypothesis. On an otherwise ordinary day sixty-five million years ago, an asteroid six miles wide collided with the earth. Exploding on contact, it released energy on the order of a hundred million megatons of TNT, or more than a million of the most powerful H-bombs ever tested. Debris, including iridium from the pulverized asteroid, spread around the globe. Day turned to night, and temperatures plunged. A mass extinction ensued.

The Alvarezes wrote up the results from Gubbio and Stevns Klint and sent them, along with their proposed explanation, to Science. “I can remember working very hard to make that paper just as solid as it could possibly be,” Walter told me.”

“THE Alvarezes’ paper, “Extraterrestrial Cause for the Cretaceous-Tertiary Extinction,” was published in June 1980. It generated lots of excitement, much of it beyond the bounds of paleontology. Journals in disciplines ranging from clinical psychology to herpetology reported on the Alvarezes’ findings, and soon the idea of an end-Cretaceous asteroid was picked up by magazines like Time and Newsweek. One commentator observed that “to connect the dinosaurs, creatures of interest to but the veriest dullards, with a spectacular extraterrestrial event” seemed “like one of those plots a clever publisher might concoct to guarantee sales.” Inspired by the impact hypothesis, a group of astrophysicists led by Carl Sagan decided to try to model the effects of an all-out war and came up with the concept of “nuclear winter,” which, in turn, generated its own wave of media coverage.

But among professional paleontologists, the Alvarezes’ idea and in many cases the Alvarezes themselves were reviled. “The apparent mass extinction is an artifact of statistics and poor understanding of the taxonomy,” one paleontologist told the New York Times.”

“But among professional paleontologists, the Alvarezes’ idea and in many cases the Alvarezes themselves were reviled. “The apparent mass extinction is an artifact of statistics and poor understanding of the taxonomy,” one paleontologist told the New York Times.

“The arrogance of those people is unbelievable,” a second asserted. “They know next to nothing about how real animals evolve, live, and become extinct. But despite their ignorance, the geochemists feel that all you have to do is crank up some fancy machine and you’ve revolutionized science.”

“Unseen bolides dropping into an unseen sea are not for me,” a third declared.”

“The Cretaceous extinctions were gradual and the catastrophe theory is wrong,” yet another paleontologist stated. But “simplistic theories will continue to come along to seduce a few scientists and enliven the covers of popular magazines.” Curiously enough, the Times’ editorial board decided to weigh in on the matter. “Astronomers should leave to astrologers the task of seeking the cause of earthly events in the stars,” the paper admonished.

To understand the vehemence of this reaction, it helps to go back, once again, to Lyell. In the fossil record, mass extinctions stand out, so much so that the very language that’s used to describe earth’s history is derived from them. In 1841, John Phillips, a contemporary of Lyell’s who succeeded him as president of the Geological Society of London, divided life into three chapters. He called the first the Paleozoic, from the Greek for “ancient life,” the second the Mesozoic, meaning “middle life,” and the third the Cenozoic, “new life.” Phillips fixed as the dividing point between the Paleozoic and the Mesozoic what would now be called the end-Permian extinction, and between the Mesozoic and the Cenozoic, the end-Cretaceous event. (In geologic parlance, the Paleozoic, Mesozoic, and Cenozoic are “eras,” and each era comprises several “periods”; the Mesozoic, for example, spans the Triassic, the Jurassic, and the Cretaceous.) The fossils from the three eras were so different that Phillips thought they represented distinct acts of creation.

“Lyell was well aware of these breaks in the fossil record. In the third volume of the Principles of Geology, he noted a “chasm” between the plants and animals found in rocks from the late Cretaceous period and those found directly above, at the start of the Tertiary period (which is now technically known as the beginning of the Paleogene). For instance, late Cretaceous deposits contained the remains of numerous species of belemnites—squid-like creatures that left behind fossils shaped like bullet casings. But belemnite fossils were never found in more recent deposits. The same pattern held for ammonites, and for “rudist bivalves—mollusks that formed immense reefs. (Rudists have been described as oysters pretending to be corals.) To Lyell, it was simply impossible, or “unphilosophical,” to imagine that this “chasm” represented what it seemed to—sudden and dramatic global change. So, in a rather neat bit of circular reasoning, he asserted that the faunal gap was just a gap in the fossil record. After comparing the life forms on both sides of the supposed gap, Lyell concluded that the unaccounted-for interval must have been a long one, roughly equivalent to all the time that had passed since the record had resumed. Using today’s dating methods, the lacuna he was positing amounts to some sixty-five million years.”

“Darwin, too, was well informed about the discontinuity at the end of the Cretaceous. In the Origin, he observed that the disappearance of the ammonites seemed to be “wonderfully sudden.” And, just like Lyell, he dismissed the ammonites and what they seemed to be saying. “For my part,” he observed,”

“I look at the natural geological record, as a history of the world imperfectly kept, and written in a changing dialect; of this history we possess the last volume alone, relating only to two or three countries. Of this volume, only here and there a short chapter has been preserved; and of each page, only here and there a few lines.”

“The fragmentary nature of the record meant that the semblance of abrupt change was just that: “With respect to the apparently sudden extermination of whole families or orders,” it must be remembered, he wrote, that “wide intervals of time” were probably unaccounted for. Had the evidence of these intervals not been lost, it would have shown “much slow extermination.” In this way, Darwin continued the Lyellian project of turning the geologic evidence on its head. “So profound is our ignorance, and so high our presumption, that we marvel when we hear of the extinction of an organic being; and as we do not see the cause, we invoke cataclysms to desolate the world!” he declared.”

“Darwin’s successors inherited the “much slow extermination” problem. The uniformitarian view precluded sudden or sweeping change of any kind. But the more that was learned about the fossil record, the more difficult it was to maintain that an entire age, spanning tens of millions of years, had somehow or other gone missing. This growing tension led to a series of increasingly tortured explanations. Perhaps there had been some sort of “crisis” at the close of the Cretaceous, but it had to have been a very slow crisis. Maybe the losses at the end of the period did constitute a “mass extinction.” But mass extinctions were not to be confused with “catastrophes.” The same year that the Alvarezes published their paper in Science, George Gaylord Simpson, at the time probably the world’s most influential paleontologist, wrote that the “turnover” at the end of the Cretaceous should be regarded as part of “a long and essentially continuous process.”

In the context of “hard-core uniformitarianism,” the impact hypothesis was worse than wrong. The Alvarezes were claiming to explain an event that hadn’t happened—one that couldn’t have happened. It was like peddling patent medicine for a fictitious illness. A few years after father and son published their hypothesis, an informal survey was conducted at a meeting of the Society of Vertebrate Paleontology. A majority of those surveyed said they thought some sort of cosmic collision might have taken place. But only one in twenty thought it had anything to do with the extinction of the dinosaurs. One paleontologist at the meeting labeled the Alvarez hypothesis “codswallop.”

“MEANWHILE, evidence for the hypothesis continued to accumulate.

The first independent corroboration came in the form of tiny grains of rock known as “shocked quartz.” Under high magnification, shocked quartz exhibits what look like scratch marks, the result of bursts of high pressure that deform the crystal structure. Shocked quartz was first noted at nuclear test sites and subsequently found in the immediate vicinity of impact craters. In 1984, grains of shocked quartz were discovered in a layer of clay from the Cretaceous-Tertiary, or K-T, boundary in eastern Montana. (K is used as the abbreviation for Cretaceous because C was already taken by the Carboniferous; today, the border is formally known as the Cretaceous-Paleogene, or K-Pg, boundary.)”

“The next clue showed up in south Texas, in a curious layer of end-Cretaceous sandstone that seemed to have been produced by an enormous tsunami. It occurred to Walter Alvarez that if there had been a giant, impact-induced tsunami, it would have scoured away shorelines, leaving behind a distinctive fingerprint in the sedimentary record. He scanned the records of thousands of sediment cores that had been drilled in the oceans, and found such a fingerprint in cores from the Gulf of Mexico. Finally, a hundred-mile-wide crater was discovered or, more accurately, rediscovered, beneath the Yucatán Peninsula. Buried under half a mile of newer sediment, the crater had shown up in gravity surveys taken in the nineteen-fifties by Mexico’s state-run oil company. Company geologists had interpreted it as the traces of an underwater volcano and, since volcanoes don’t yield oil, promptly forgotten about it. When the Alvarezes went looking for cores the company had drilled in the area, they were told that they’d been destroyed in a fire; really, though, they had just been misplaced. The cores were finally located in 1991 and found to contain a layer of glass—rock that had melted “, then rapidly cooled—right at the K-T boundary. To the Alvarez camp, this was the clincher, and it was enough to move many uncommitted”scientists into the pro-impact column. “Crater supports extinction theory,” the Times announced. By this point, Luis Alvarez had died of complications from esophageal cancer. Walter dubbed the formation the “Crater of Doom.” It became more widely known, after the nearest town, as the Chicxulub crater.”

“Those eleven years seemed long at the time, but looking back they seem very brief,” Walter told me. “Just think about it for a moment. Here you have a challenge to a uniformitarian viewpoint that basically every geologist and paleontologist had been trained in, as had their professors and their professors’ professors, all the way back to Lyell. And what you saw was people looking at the evidence. And they gradually did come to change their minds.”

“WHEN the Alvarezes published their hypothesis, they knew of only three sites where the iridium layer was exposed: the two Walter had visited in Europe and a third, which they’d been sent samples from, in New Zealand. In the decades since, dozens more have been located, including one near a nude beach in Biarritz, another in the Tunisian desert, and a third in suburban New Jersey. Neil Landman, a paleontologist who specializes in ammonites, often takes field trips to this last site, and one warm fall day I invited myself to tag along. We met in front of the American Museum of Natural History, in Manhattan, where Landman has his office in a turret overlooking Central Park, and, together with a pair of graduate students, headed south to the Lincoln Tunnel.

Driving through northern New Jersey, we passed a succession of strip malls and car dealerships that seemed to repeat every few miles, like dominoes. Eventually, in the general vicinity of Princeton, we pulled into a parking lot next to a baseball field.

“(Landman would prefer that I not reveal the exact location of the field, for fear of attracting fossil collectors.) In the parking lot, we met up with a geologist named Matt Garb, who teaches at Brooklyn College. Garb, Landman, and the graduate students shouldered their gear. We circumnavigated the baseball field—empty in the middle of a school day—and struck out through the underbrush. Soon we reached a shallow creek. Its banks were covered in rust-colored slime. Brambles hung over the water. Fluttering from these were tattered banners of debris: lost plastic bags, scraps of newspaper, the rings from ancient six-packs. “To me, this is better than Gubbio,” Landman announced.”

“During the late Cretaceous, he explained to me, the park, the creek bed, and everything around us for many miles would have been under water. At that point, the world was very warm—lush forests grew in the Arctic—and sea levels were high. Most of New Jersey formed part of the continental shelf of what’s now eastern North America, which, as the Atlantic was then much narrower, was considerably closer to what’s now Europe. Landman pointed to a spot in the creek bed a few inches above the water line. There, he told me, was the iridium layer. Although it wasn’t in any way visibly different, Landman knew where it was because he’d had the sequence analyzed a few years earlier. Landman is stocky, with a wide face and a graying beard. He had dressed for the trip in khaki shorts and old sneakers. “He waded into the creek to join the others, who were already hacking at the bed with their pickaxes. Soon, someone found a fossilized shark’s tooth. Someone else dug out a piece of an ammonite. It was about the size of a strawberry and covered in little pimples, or tubercles. Landman identified it as belonging to the species Discoscaphites iris.”

“AMMONITES floated through the world’s shallow oceans for more than three hundred million years, and their fossilized shells turn up all around the world. Pliny the Elder, who died in the eruption that buried Pompeii, was already familiar with them, although he considered them to be precious stones. (The stones, he related in his Natural History, were said to bring prophetic dreams.) In medieval England, ammonites were known as “serpent stones,” and in Germany they were used to treat sick cows. In India, they were—and to a certain extent still are—revered as manifestations of Vishnu.”

“Like nautiluses, to whom they were distantly related, ammonites constructed spiral shells divided into multiple chambers. The animals themselves occupied only the last and largest chamber; the rest were filled with air, an arrangement that might be compared to an apartment building in which just the penthouse is rented. The walls between the chambers, known as septa, were fantastically elaborate, folded into intricate ruffles, “like the edges of a snowflake. (Individual species can be identified by the distinctive patterns of their pleats.) This evolutionary development allowed ammonites to build shells that were at once light and robust—capable of withstanding many atmospheres’ worth of water pressure. Most ammonites could fit in a human hand; some grew to be the size of kiddie pools.”

“Based on the number of teeth ammonites had—nine—it’s believed that their closest living kin are octopuses. But since ammonites’ soft body parts are virtually never preserved, what exactly the animals looked like and how they lived are largely matters of inference. It’s probable, though not certain, that they propelled themselves by shooting out a jet of water, which means that they could only travel backward.

”

“I remember when I was a kid taking paleontology, and I learned that pterodactyls could fly,” Landman told me. “My immediate question was, well, how high could they fly? And it’s hard to come up with those numbers.”

“I’ve studied ammonites for forty years, and I’m still not sure exactly what they liked,” he went on. “I feel they liked water twenty, thirty, maybe forty meters deep. They were swimmers “but not very good swimmers. I think they lived a quiet existence.” In drawings, ammonites are usually depicted as resembling squids that have been stuffed into snail shells. Landman, however, has trouble with this depiction. He believes that ammonites, though commonly shown with several streaming tentacles, in fact had none. In a drawing that accompanies a recent journal article he published in the journal Geobios, ammonites are shown looking like little more than blobs. They have stubby armlike appendages, which are arrayed in a circle and connected by a web of tissue. In males, one of the arms pokes up out of the webbing to form the cephalopod version of a penis.”

“Landman attended graduate school at Yale in the nineteen-seventies. As a student in the pre-Alvarez days, he was taught that ammonites were declining throughout the Cretaceous, so their eventual disappearance was nothing to get too worked up about. “The sense was, oh, you know, the ammonites were just dying out,” he recalled. Subsequent discoveries, many of them made by Landman himself, have shown that, on the contrary, ammonites were doing just fine.”

“Here you have lots of species, and we’ve collected thousands of specimens over the last few years,” he told me over the clank of the others’ pickaxes. Indeed, in the creek bed, Landman recently came upon two entirely new species of ammonite. One of these he named, in honor of a colleague, Discoscaphites minardi. The other he named, in honor of the place, Discoscaphites jerseyensis. Discoscaphites jerseyensis probably had little spines poking out of its shell, which, Landman speculates, helped the animal appear larger and more intimidating than it actually was.”

“IN their original paper, the Alvarezes proposed that the main cause of the K-T mass extinction was not the impact itself or even the immediate aftermath. The truly catastrophic effect of the asteroid—or, to use the more generic term, bolide—was the dust. In the intervening decades, this account has been subjected to numerous refinements. (The date of the impact has also been pushed back—to sixty-six million years ago.) Though scientists still vigorously argue about many of the details, one version of the event runs as follows:”

“The bolide arrived from the southeast, traveling at a low angle relative to the earth, so that it came in not so much from above as from the side, like a plane losing altitude. When it slammed into the Yucatán Peninsula, it was moving at something like forty-five thousand miles per hour, and, due to its trajectory, North America was particularly hard-hit. A vast cloud of searing vapor and debris raced over the continent, expanding as it moved and incinerating anything in its path. “Basically, if you were a triceratops in Alberta, you had about two minutes before you got vaporized” is how one geologist put it to me.”

“n the process of excavating the enormous crater, the asteroid blasted into the air more than fifty times its own mass in pulverized rock. As the ejecta fell back through the atmosphere, the particles incandesced, lighting the sky everywhere at once from directly overhead and generating enough heat to, in effect, broil the surface of the planet. Owing to the composition of the Yucatán Peninsula, the dust thrown up was rich in sulfur. Sulfate aerosols are particularly effective at blocking sunlight, which is the reason a single volcanic eruption, like Krakatoa, can depress global temperatures for years. After the initial heat pulse, the world experienced a multiseason “impact winter. “Forests were decimated. Palynologists, who study ancient spores and pollen, have found that diverse plant communities were replaced entirely by rapidly dispersing ferns. (This phenomenon has become known as the “fern spike.”) Marine ecosystems effectively collapsed, and they remained in that state for at least half a million, and perhaps as many as several million, years. (The desolate post-impact sea has been dubbed the “Strangelove ocean.”)

It’s impossible to give anything close to a full account of the various species, genera, families, and even whole orders that went extinct at the K-T boundary. On land, every animal larger than a cat seems to have died out. The event’s most famous victims, the dinosaurs—or, to be more precise, the non-avian dinosaurs—suffered a hundred percent losses. “Among the groups that were probably alive right up to the end of the Cretaceous were such familiar museum shop fixtures as hadrosaurs, ankylosaurs, tyrannosauruses, and triceratops. (The cover of Walter Alvarez’s book on the extinction, T. Rex and the Crater of Doom, shows an angry-looking tyrannosaurus reacting with horror to the impact.) Pterosaurs, too, disappeared. Birds were also hard-hit; perhaps three-quarters of all bird families, perhaps more, went extinct. Enantiornithine birds, which retained such archaic features as teeth, were wiped out, as were Hesperornithine birds, which were aquatic and for the most part flightless. “The same goes for lizards and snakes; around four-fifths of all species vanished. Mammals’ ranks, too, were devastated; something like two-thirds of the mammalian families living at the end of the Cretaceous disappear at the boundary.”

“In the sea, plesiosaurs, which Cuvier had at first found implausible and then “monstrous,” died out. So did mosasaurs, belemnites, and, of course, ammonites. Bivalves, familiar to us today in the form of mussels and oysters, suffered heavy casualties, as did brachiopods, which look like clams but have a totally different anatomy, and bryozoans, which look like corals but once again are totally unrelated. Several groups of marine microorganisms came within a micron or two of annihilation. Among planktonic foraminifera, something like ninety-five percent of all species disappeared, including Abathomphalus mayaroensis, whose remains are found in the last layer of Cretaceous limestone in Gubbio. (Planktonic foraminifera live near the ocean surface; benthic species live on the ocean floor.)

In general, the more that’s been learned about the K-T boundary, the more wrongheaded Lyell’s reading of the fossil record appears. The problem with the record is not that slow extinctions appear abrupt. It’s that even abrupt extinctions are likely to look protracted.”

“Consider the accompanying diagram. Every species has what is known as a “preservation potential”—the odds that an individual of that species will become fossilized—and this varies depending on, among other things, how common the animal is, where it lives, and what it’s made out of. (Thick-shelled marine organisms have a much better chance of being preserved than, say, birds with hollow bones.)”

“In this diagram, the large white circles represent species that are rarely fossilized, the medium-sized circles those that are preserved more frequently, and the small white dots species that are more abundant still. Even if all of these species died out at exactly the same moment, it would appear that the white-circle species had vanished much earlier, simply because its remains are rarer. This effect—known as the Signor-Lipps effect, after the scientists who first identified it—tends to “smear out” sudden extinction events, making them look like long, drawn-out affairs.”

“Following the K-T extinction, it took millions of years for life to recover its former level of diversity. In the meantime, many surviving taxa seem to have shrunk. This phenomenon, which can be seen in the very tiny forams that show up above the iridium layer at Gubbio, is called the Lilliput effect.”

“LANDMAN, Garb, and the graduate students chipped away at the creek bed all morning. Although we were in the middle of the country’s most densely populated state, not a single person passed by to wonder at what we were doing. As the day grew warmer and more humid, it was pleasant to stand ankle-deep in the water (though I did wonder about the reddish slime). Someone had brought along an empty cardboard box, and, since I didn’t have a pickax, I helped out by gathering up the fossils the others had found and arranging them in the box. “Several more bits of Discoscaphites iris turned up, as well as pieces of an ammonite, Eubaculites carinatus, which, instead of having a spiral shell, had one that was long and slender and shaped like a spear. (One theory of the ammonites’ demise, popular in the early part of the twentieth century, was that the uncoiled shells of species like Eubaculites carinatus indicated that the group had exhausted its practical possibilities and entered some sort of decadent, Lady Gaga-ish phase.) At one point, Garb rushed over in a flurry of excitement. He was carrying a fist-sized chunk of the creek “bed and pointed out to me, along one edge, what looked like a tiny fingernail. This, he explained, was a piece of an ammonite’s jaw. Ammonite jaws are more common than other body parts but still extremely rare.”

“It was worth the trip just for that,” he exclaimed.

It’s unclear what aspect of the impact—the heat, the darkness, the cold, the change in water chemistry—did in the ammonites. Nor is it entirely clear why some of their cephalopod cousins survived. In contrast to ammonites, nautiluses, for example, sailed through the extinction event: pretty much all of the species known from the end of the Cretaceous survived into the Tertiary.”

“One theory of the disparity starts with eggs. Ammonites produced very tiny eggs, only a few hundredths of an inch across. The resulting hatchlings, or ammonitellae, had no means of locomotion; they just floated near the surface of the water, drifting along with the current. Nautiluses, for their part, lay very large eggs, among the largest of all invertebrates, nearly an inch in diameter. Hatchling nautiluses emerge, after nearly a year’s gestation, as miniature adults and then immediately start swimming around, searching for food in the depths. Perhaps in the aftermath of the impact, conditions at the ocean surface were so toxic that ammonitellae could not survive, while lower down in the water column the situation was less dire, so juvenile nautiluses managed to endure.”

“Whatever the explanation, the contrasting fate of the two groups raises a key point. Everything (and everyone) alive today is descended from an organism that somehow survived the impact. But it does not follow from this that they (or we) are any better adapted. In times of extreme stress, the whole concept of fitness, at least in a Darwinian sense, loses its meaning: how could a creature be adapted, either well or ill, for conditions it has never before encountered in its entire evolutionary history? At such moments, what Paul Taylor, a paleontologist at London’s Natural History Museum, calls “the rules of the survival game” abruptly change. Traits that for many millions of years were advantageous all of a sudden become lethal (though it may be difficult, millions of years after the fact, to identify just what those traits were). And what holds for ammonites and nautiluses applies equally well to belemnites and squids, plesiosaurs and turtles, dinosaurs and mammals. The reason this book is being written by a hairy biped, rather than a scaly one, has more to do with dinosaurian misfortune than with any particular mammalian virtue.”

“There’s nothing ammonites were doing wrong,” Landman told me as we packed up the last fossils from the creek and prepared to head back to New York. “Their hatchlings would have been like plankton, which for all of their existence would have been terrific. What better way to get around and distribute the species? Yet here, in the end, it may well have been their undoing.”

“CHAPTER V

WELCOME TO THE ANTHROPOCENE

Dicranograptus ziczac

“In 1949, a pair of Harvard psychologists recruited two dozen undergraduates for an experiment about perception. The experiment was simple: students were shown playing cards and asked to identify them as they flipped by. Most of the cards were perfectly ordinary, but a few had been doctored, so that the deck contained, among other oddities, a red six of spades and a black four of hearts. When the cards went by rapidly, the students tended to overlook the incongruities; they would, for example, assert that the red six of spades was a six of hearts, or call the black four of hearts a four of spades. When the cards went by more slowly, they struggled to make sense of what they were more slowly, they struggled to make sense of what they were seeing. Confronted with a red spade, some said it looked “purple” or “brown” or “rusty black.” Others were completely flummoxed.”

“The symbols “look reversed or something,” one observed.

“I can’t make the suit out, whatever it is,” another exclaimed. “I don’t know what color it is now or whether it’s a spade or heart. I’m not even sure now what a spade looks like! My God!”

The psychologists wrote up their findings in a paper titled “On the Perception of Incongruity: A Paradigm.” Among those who found this paper intriguing was Thomas Kuhn. To Kuhn, the twentieth century’s most influential historian of science, the experiment was indeed paradigmatic: it revealed how people process disruptive information. Their first impulse is to force it into a familiar framework: hearts, spades, clubs. Signs of mismatch are disregarded for as long as possible—the red spade looks “brown” or “rusty.” At the point the anomaly becomes simply too glaring, a crisis ensues—what the psychologists dubbed the “’My God!’ reaction.”

“This pattern was, Kuhn argued in his seminal work, The Structure of Scientific Revolutions, so basic that it shaped not only individual perceptions but entire fields of inquiry. Data that did not fit the commonly accepted assumptions of a discipline would either be discounted or explained away for as long as possible. The more contradictions accumulated, the more convoluted the rationalizations became. “In science, as in the playing card experiment, novelty emerges only with difficulty,” Kuhn wrote. But then, finally, someone came along who was willing to call a red spade a red spade. Crisis led to insight, and the old framework gave way to a new one. This is how great scientific discoveries or, to use the term Kuhn made so popular, “paradigm shifts” took place.”

“The history of the science of extinction can be told as a series of paradigm shifts. Until the end of the eighteenth century, the very category of extinction didn’t exist. The more strange bones were unearthed—mammoths, Megatherium, mosasaurs—the harder naturalists had to squint to fit them into a familiar framework. And squint they did. The giant bones belonged to elephants that had been washed north, or hippos that had wandered west, or whales with malevolent grins. When Cuvier arrived in Paris, he saw that the mastodon’s molars could not be fit into the established framework, a “My God” moment that led him to propose a whole new way of seeing them. Life, Cuvier recognized, had a history. This history was marked by loss and punctuated by events too terrible for human imagining. “Though the world does not change with a change of paradigm, the scientist afterward works in a different world” is how Kuhn put it.”

“In his Recherches sur les ossemens fossiles, Cuvier listed dozens of espèces perdues, and he felt sure there were more awaiting discovery. Within a few decades, so many extinct creatures had been identified that Cuvier’s framework began to crack. To keep pace with the growing fossil record, the number of disasters had to keep multiplying. “God knows how many catastrophes” would be needed, Lyell scoffed, poking fun at the whole endeavor. Lyell’s solution was to reject catastrophe altogether. In Lyell’s—and later Darwin’s—formulation, extinction was a lonely affair. Each species that had vanished had shuffled off all on its own, a victim of the “struggle for life” and its own defects as a “less improved form.”

“The uniformitarian account of extinction held up for more than a century. Then, with the discovery of the iridium layer, science faced another crisis. (According to one historian, the Alvarezes’ work was “as explosive for science as an impact would have been for earth.”) The impact hypothesis dealt with a single moment in time—a terrible, horrible, no-good day at the end of the Cretaceous. But that single moment was enough to crack the framework of Lyell and Darwin. Catastrophes did happen.”

“What is sometimes labeled neocatastrophism, but is mostly nowadays just regarded as standard geology, holds that conditions on earth change only very slowly, except when they don’t. In this sense the reigning paradigm is neither Cuvierian nor Darwinian but combines key elements of both—“long periods of boredom interrupted occasionally by panic.” Though rare, these moments of panic are disproportionately important. They determine the pattern of extinction, which is to say, the pattern of life.”

“THE path leads up a hill, across a fast-moving stream, back across the stream, and past the carcass of a sheep, which, more than just dead, looks deflated, like a lost balloon. The hill is bright green but treeless; generations of the sheep’s aunts and uncles have kept anything from growing much above muzzle-height. In my view, it’s raining. Here in the Southern Uplands of Scotland, though, I’m told by one of the geologists I’m hiking with, this counts only as a light drizzle, or smirr.”

“Our goal is a spot called Dob’s Linn, where, according to an old ballad, the Devil himself was pushed over a precipice by a pious shepherd named Dob. By the time we reach the cliff, the smirr seems to be smirring harder. There’s a view over a waterfall, which crashes down into a narrow valley. A few yards farther up the path there’s a jagged outcropping of rock, which is striped vertically, like an umpire’s jersey, in bands of light and dark. Jan Zalasiewicz, a stratigrapher from the University of Leicester, sets his rucksack down on the soggy ground and adjusts his red rain jacket. He points to one of the light-colored stripes. “Bad things happened in here,” he tells me.”

“The rocks that we are looking at date back some 445 million years, to the last part of the Ordovician period. At that point, the globe was experiencing a continental logjam; most of the land—including what’s now Africa, South America, Australia, and Antarctica—was joined into one giant mass, Gondwana, which spanned more than ninety degrees latitude. England belonged to the continent—now lost—of Avalonia, and Dob’s Linn lay in the Southern Hemisphere, at the bottom of an ocean known as the Iapetus.”

“The Ordovician period followed directly after the Cambrian, which is known, even to the most casual of geology students, for the “explosion” of new life forms that appeared.* The Ordovician, too, was a time when life took off excitedly in new directions—the so-called Ordovician radiation—though it remained, for the most part, still confined to the water. During the Ordovician, the number of marine families tripled, and the seas filled with creatures we would more or less recognize (the progenitors of today’s starfish and sea urchins and snails and nautiluses) and also plenty that we would not (conodonts, which probably were shaped like eels; trilobites, which sort of resembled horseshoe crabs; and giant sea scorpions, which, as best as can be determined, looked like something out of a nightmare).”

“The first reefs appeared, and the ancestors of today’s clams took on their clam-like form. Toward the middle of the Ordovician, the first plants began to colonize the land. These were very early mosses or liverworts, and they clung low to the ground, as if not quite sure what to make of their new surroundings.”

“At the end of the Ordovician, some 444 million years ago, the oceans emptied out. Something like eighty-five percent of marine species died off. For a long time, the event was regarded as one of those pseudo-catastrophes that just went to show how little the fossil record could be trusted. Today, it’s seen as the first of the Big Five extinctions, and it’s thought to have taken place in two brief, intensely deadly pulses. Though its victims are nowhere near as charismatic as those taken out at the end of “the Cretaceous, it, too, marks a turning point in life’s history—a moment when the rules of the game suddenly flipped, with consequences that, for all intents and purposes, will last forever.”

“Those animals and plants that made it through the Ordovician extinction “went on to make the modern world,” the British paleontologist Richard Fortey has observed. “Had the list of survivors been one jot different, then so would the world today.”

“ZALASIEWICZ—MY guide at Dob’s Linn—is a slight man with shaggy hair, pale blue eyes, and a pleasantly formal manner. He is an expert on graptolites, a once vast and extremely diverse class of marine organisms that thrived during the Ordovician and then, in the extinction event, were very nearly wiped out. To the naked eye, graptolite fossils look like scratches or in some cases tiny petroglyphs. (The word “graptolite” comes from the Greek meaning “written rock”; it was coined by Linnaeus, who dismissed graptolites as mineral encrustations trying to pass themselves off as the remnants of animals.) Viewed through a hand lens, they often prove to have lovely, evocative shapes; one species suggests a feather, another a lyre, a third the frond of a fern. “Graptolites were colonial animals; each individual, known as a zooid, built itself a tiny, tubular shelter, known as a theca, which was attached to its neighbor’s, like a row house. A single graptolite fossil thus represents a whole community, which drifted or more probably swam along as a single entity, feeding off even smaller plankton. No one knows exactly what the zooids looked like—as with ammonites, the creatures’ soft parts resist preservation—but graptolites are now believed to be related to pterobranchs, a small and hard-to-find class of living marine organisms that resemble Venus flytraps.”

“Graptolites had a habit—endearing from a stratigrapher’s point of view—of speciating, spreading out, and dying off, all in relatively short order. Zalasiewicz compares them to Natasha, “the tender heroine of War and Peace. They were, he says, “delicate, nervous, and very sensitive to things around them.” This makes them useful index fossils—successive species can be used to identify successive layers of rock.”

“Finding graptolites at Dob’s Linn turns out, even for the most amateur of collectors, to be easy. The dark stone in the jagged outcropping is shale. It takes only a gentle hammer-tap to dislodge a chunk. Another tap splits the chunk laterally. It divides like a book opening to a well-thumbed page. Often on the stony surface there’s nothing to see, but just as often there’s one (or more) faint marks—messages from a former world. One of the graptolites I happen across has been preserved with peculiar clarity. It’s shaped like a set of false eyelashes, but very small, as if for a Barbie. Zalasiewicz tells me—doubtless exaggerating—that I have found a “museum quality specimen.” I pocket it.”

“Once Zalasiewicz shows me what to look for, I, too, can make out the arc of the extinction. In the dark shales, graptolites are plentiful and varied. Soon I’ve collected so many, the pockets of my jacket are sagging. Many of the fossils are variations on the letter V, with two arms branching away from a central node. Some look like zippers, others like wishbones. Still others have arms growing off their arms like tiny trees.”

“The lighter stone, by contrast, is barren. There’s barely a graptolite to be found in it. The transition from one state to another—from black stone to gray, from many graptolites to almost none—appears to have occurred suddenly and, according to Zalasiewicz, did occur suddenly.”

“The change here from black to gray marks a tipping point, if you like, from a habitable sea floor to an uninhabitable one,” he tells me. “And one might have seen that in the span of a human lifetime.” He describes this transition as distinctly “Cuvierian.”

“Two of Zalasiewicz’s colleagues, Dan Condon and Ian Millar, of the British Geological Survey, have made the hike with us out to Dob’s Linn. The pair are experts in isotope chemistry and are planning to collect samples from each of the stripes in the outcropping—samples they hope will contain tiny crystals of zircon. Once back at the lab, they will dissolve the crystals and run the results through a mass spectrometer. This will allow them to say, give or take half a million years or so, when each of the layers was formed. Millar is Scottish and claims to be undaunted by the smirr. “Eventually, though, even he has to acknowledge that, in English, it’s pouring. Rivulets of mud are running down the face of the outcropping, making it impossible to get clean samples. It is decided that we will try again the following day. The three geologists pack up their gear, and we squish back down the trail to the car. Zalasiewicz has made reservations at a bed-and-breakfast in the nearby town of Moffat, whose attractions, I have read, include the world’s narrowest hotel and a bronze sheep.”

“Once everyone has changed into dry clothes, we meet in the sitting room of the B & B for tea. Zalasiewicz has brought along several recent publications of his on graptolites. Settling back in their chairs, Condon and Millar roll their eyes. Zalasiewicz ignores them, patiently explaining to me the import of his latest monograph, “Graptolites in British Stratigraphy,” which runs sixty-six single-spaced pages and includes detailed illustrations of more than 650 species. In the monograph, the effects of the extinction show up more systematically, if also less vividly than on the rain-slicked hillside. Until the end of the Ordovician, V-shaped graptolites dominated. “ These included species like Dicranograptus ziczac, whose tiny cups were arranged along arms that curled away and then toward each other, like tusks, and Adelograptus divergens, which, in addition to its two main arms, had little side-arms that stuck out like thumbs. “Only a handful of graptolite species survived the extinction event; eventually, these diversified and repopulated the seas in the Silurian. But Silurian graptolites had a streamlined body plan, more like a stick than a set of branches. The V-shape had been lost, never to reappear. Here writ very, very small is the fate of the dinosaurs, the mosasaurs, and the ammonites—a once highly successful form relegated to oblivion.”

“WHAT happened 444 million years ago to nearly wipe out the graptolites, not to mention the conodonts, the brachiopods, the echinoderms, and the trilobites?”

“In the years immediately following the publication of the Alvarez hypothesis, it was generally believed—at least among those who considered the hypothesis more than “codswallop”—that a unified theory of mass extinction was at hand. If an asteroid had produced one “chasm” in the fossil record, it seemed reasonable to expect that impacts had caused all of them. This idea received a boost in 1984, when a pair of paleontologists from the University of Chicago published a comprehensive analysis of the marine fossil record.”

“The study revealed that in addition to the five major mass extinctions, there had been “many lesser extinction events. When all of these were considered together, a pattern emerged: mass extinctions seemed to take place at regular intervals of roughly twenty-six million years. Extinction, in other words, occurred in periodic bursts, like cicadas crawling out of the earth. The two paleontologists, David Raup and Jack Sepkoski, were unsure what had caused these bursts, but their best guess was some “astronomical and astrophysical cycle,” having to do with “the passage of our solar system through the spiral arms of the Milky Way. A group of astrophysicists—as it happened, colleagues of the Alvarezes at Berkeley—took the speculation one step farther. The periodicity, the group argued, could be explained by a small “companion star” to the sun, which, every twenty-six million years, passed through the Oort cloud, producing comet showers that rained destruction on the earth. The fact that no one had ever seen this star, dubbed with horror-movie flair “Nemesis,” was, to the Berkeley group, a problem, but not an insurmountable one; there were plenty of small stars out there, still waiting to be cataloged.”

“In the popular media, what became known as the “Nemesis Affair” generated almost as much excitement as the original asteroid hypothesis. (One reporter described the story as having everything but sex and the royal family.) Time ran a cover article, which was soon followed by another disapproving editorial in the New York Times. (The editorial pooh-poohed the notion of a “mysterious death-star.”) This time, the newspaper was onto something. Though the Berkeley group spent the next year or so scanning the heavens for Nemesis, no glimmer of a “death star” was discovered. More significantly, upon further analysis, the evidence for periodicity began to fall apart. “If there’s a consensus, it’s that what we were seeing was a statistical fluke,” David Raup told me.”

“Meanwhile, the search for iridium and other signs of extraterrestrial impacts was faltering. Together with many others, Luis Alvarez had thrown himself into this hunt. At a time when scientific collaboration with the Chinese was practically unheard of, he’d managed to obtain rock samples from southern China that spanned the boundary between the Permian and Triassic periods. The end-Permian or Permo-Triassic extinction was the biggest of the Big Five, an episode that came scarily close to eliminating multicellular life altogether. Luis was thrilled to find a layer of clay nestled between the bands of rock from southern China, just as there had been at Gubbio. “We felt sure that there would be lots of iridium there,” he would later recall. But the Chinese clay turned out to be, chemically speaking, mundane, its iridium content too infinitesimal to be measured. Higher-than-normal iridium levels were subsequently detected at the end of the Ordovician, in rocks from, among other places, Dob’s Linn. ”

“However, none of the other telltale signs of an impact, such as shocked quartz, turned up in the right time frame, and “it was determined that the elevated iridium levels were more plausibly—if less spectacularly—attributed to the vagaries of sedimentation”

“The current theory is that the end-Ordovician extinction was caused by glaciation. For most of the period, a so-called greenhouse climate prevailed—carbon dioxide levels in the air were high and so, too, were sea levels and temperatures. But right around the time of the first pulse of extinction—the one that wreaked havoc among the graptolites—CO2 levels dropped. Temperatures fell and Gondwana froze. Evidence of the Ordovician glaciation has been found in such far-flung remnants of the supercontinent as Saudi Arabia, Jordan, and Brazil. Sea levels plummeted, and many marine habitats were eliminated, presumably to the detriment of marine organisms. The oceans’ chemistry changed, too; among other things, colder water holds more oxygen. “No one is sure whether it was the temperature change or one of the many knock-on effects that killed the graptolites; as Zalasiewicz put it to me, “You have a body in the library, and a half a dozen butlers wandering around, looking sheepish.” Nor does anyone know what caused the change to begin with. One theory has it that the glaciation was produced by the early mosses that colonized the land and, in so doing, helped draw carbon dioxide out of the air. If this is the case, the first mass extinction of animals was caused by plants.”

“The end-Permian extinction also seems to have been triggered by a change in the climate. But in this case, the change went in the opposite direction. Right at the time of extinction, 252 million years ago, there was a massive release of carbon into the air—so massive that geologists have a hard time even imagining where all the carbon could have come from. Temperatures soared—the seas warmed by as much as eighteen degrees—and the chemistry of the oceans went haywire, as if in an out-of-control aquarium. The water became acidified, and the amount of dissolved oxygen dropped so low that many organisms probably, in effect, suffocated. Reefs collapsed. “The end-Permian extinction took place, though not quite in a human lifetime, in geologic terms nearly as abruptly; according to the latest research by Chinese and American scientists, the whole episode lasted no more than two hundred thousand years, and perhaps less than a hundred thousand. By the time it was over, something like ninety percent of all species on earth had been eliminated. Even intense global warming and ocean acidification seem inadequate to explain losses on such a staggering scale, and so additional mechanisms are still being sought. One hypothesis has it that the heating of the oceans favored bacteria that produce hydrogen sulfide, which is poisonous to most other forms of life. According to this scenario, hydrogen sulfide accumulated in the water, killing off marine creatures, then it leaked into the air, killing off most everything else. The sulfate- “reducing bacteria changed the color of the oceans and the hydrogen sulfide the color of the heavens; the science writer Carl Zimmer has described the end-Permian world as a “truly grotesque place” where glassy, purple seas released poisonous bubbles that rose “to a pale green sky.”

“If twenty-five years ago it seemed that all mass extinctions would ultimately be traced to the same cause, now the reverse seems true. As in Tolstoy, every extinction event appears to be unhappy—and fatally so—in its own way. It may, in fact, be the very freakishness of the events that renders them so deadly; all of a sudden, organisms find themselves facing conditions for which they are, evolutionarily, completely unprepared.”

“I think that, after the evidence became pretty strong for the impact at the end of the Cretaceous, those of us who were working on this naively expected that we would go out and find evidence of impacts coinciding with the other events,” Walter Alvarez told me. “And it’s turned out to be much more complicated. We’re seeing right now that a mass extinction can be caused by human beings. So it’s clear that we do not have a general theory of mass extinction.”

“THAT evening in Moffat, once everyone had had enough of tea and graptolites, we went out to the pub on the ground floor of the world’s narrowest hotel. After a pint or two, the conversation turned to another one of Zalasiewicz’s favorite subjects: giant rats. Rats have followed humans to just about every corner of the globe, and it is Zalasiewicz’s professional opinion that one day they will take over the earth.”

“Some number will probably stay rat-sized and rat-shaped,” he told me. “But others may well shrink or expand. Particularly if there’s been epidemic extinction and ecospace opens up, rats may be best placed to take advantage of that. And we know that change in size can take place fairly quickly.” I recalled a rat I once watched drag a pizza crust along the tracks at an Upper West Side subway station. I imagined it waddling through a deserted tunnel blown up to the size of a Doberman.”

“Though the connection might seem tenuous, Zalasiewicz’s interest in giant rats represents a logical extension of his interest in graptolites. He is fascinated by the world that preceded humans and also—increasingly—by the world that humans will leave behind. One project informs the other. When he studies the Ordovician, he’s trying to reconstruct the distant past on the basis of the fragmentary clues that remain: fossils, isotopes of carbon, layers of sedimentary rock. When he contemplates the future, he’s trying to imagine what will remain of the present once the contemporary world has been reduced to fragments: fossils, isotopes of carbon, layers of sedimentary rock. “Zalasiewicz is convinced that even a moderately competent stratigrapher will, at the distance of a hundred million years or “so, be able to tell that something extraordinary happened at the moment in time that counts for us as today. This is the case even though a hundred million years from now, all that we consider to be the great works of man—the sculptures and the libraries, the monuments and the museums, the cities and the factories—will be compressed into a layer of sediment not much thicker than a cigarette paper. “We have already left a record that is now indelible,” Zalasiewicz has written.”

“One of the ways we’ve accomplished this is through our restlessness. Often purposefully and just as often not, humans have rearranged the earth’s biota, transporting the flora and fauna of Asia to the Americas and of the Americas to Europe and of Europe to Australia. Rats have consistently been on the vanguard of these movements, and they have left their bones scattered everywhere, including on islands so remote that humans never bothered to settle them. The Pacific rat, Rattus exulans, a native of southeast Asia, traveled with Polynesian seafarers to, among many other places, Hawaii, Fiji, Tahiti, Tonga, Samoa, Easter Island, and New Zealand. Encountering few predators, stowaway Rattus exulans multiplied into what the New Zealand paleontologist Richard Holdaway has described as “a grey tide” that turned “everything edible into rat protein.” (A recent study of pollen and animal remains on Easter Island concluded that it wasn’t humans who deforested the landscape; rather, it was the rats that came along for the ride and then bred unchecked. ”

“The native palms couldn’t produce seeds fast enough to keep up with their appetites.) When Europeans arrived in the Americas, and then continued west to the islands the Polynesians had settled, they brought with them the even-more-adaptable Norway rat, Rattus norvegicus.” “In many places, Norway rats, which are actually from China, outcompeted the earlier rat invaders and, in so doing, ravaged the bird and reptile populations the Pacific rats had missed. Rats thus might be said to have created their own “ecospace,” which their progeny seem well positioned to dominate. The descendants of today’s rats, according to Zalasiewicz, will radiate out to fill the niches that Rattus exulans and Rattus norvegicus helped empty. He imagines the rats of the future evolving into new shapes and sizes—some “smaller than shrews,” others as large as elephants. “We might,” he has written, “include among them—for curiosity’s sake and to keep our options open—a species or two of large naked rodent, living in caves, shaping rocks as primitive tools and wearing the skins of other mammals that they have killed and eaten.”

“Meanwhile, whatever the future holds for rats, the extinction event that they are helping to bring about will leave its own distinctive mark. Not yet anywhere near as drastic as the one recorded in the mudstone at Dob’s Linn or in the clay layer in Gubbio, it will nevertheless appear in the rocks as a turning point. Climate change—itself a driver of extinction—will also “leave behind geologic traces, as will nuclear fallout and river diversion and monoculture farming and ocean acidification.”

“For all of these reasons, Zalasiewicz believes that we have entered a new epoch, which has no analog in earth’s history. “Geologically,” he has observed, “this is a remarkable episode.”

“OVER the years, a number of different names have been suggested for the new age that humans have ushered in. The noted conservation biologist Michael Soulé has suggested that instead of the Cenozoic, we now live in the “Catastrophozoic” era. Michael Samways, an entomologist at South Africa’s Stellenbosch University, has floated the term “Homogenocene.” Daniel Pauly, a Canadian marine biologist, has proposed the “Myxocene,” from the Greek word for “slime,” and Andrew Revkin, an American journalist, has offered the “Anthrocene.” (Most of these terms owe their origins, indirectly at least, to Lyell, who, back in the eighteen-thirties, coined the words Eocene, Miocene, and Pliocene.)”

“The word “Anthropocene” is the invention of Paul Crutzen, a Dutch chemist who shared a Nobel Prize for discovering the effects of ozone-depleting compounds. The importance of this discovery is difficult to overstate; had it not been made—and had the chemicals continued to be widely used—the ozone “hole” that opens up every spring over Antarctica would have expanded until eventually it encircled the entire earth. (One of Crutzen’s fellow Nobelists reportedly came home from his lab one night and told his wife, “The work is going well, but it looks like it might be the end of the world.”)”

“Crutzen told me that the word “Anthropocene” came to him while he was sitting at a meeting. The meeting’s chairman kept referring to the Holocene, the “wholly recent” epoch, which began at the conclusion of the last ice age, 11,700 years ago, and which continues—at least officially—to this day.”

“Let’s stop it,’” Crutzen recalled blurting out. “’We are no longer in the Holocene; we are in the Anthropocene.’ Well, it was quiet in the room for a while.” At the next coffee break, the Anthropocene was the main topic of conversation. Someone came up to Crutzen and suggested that he patent the term.

Crutzen wrote up his idea in a short essay, “Geology of Mankind,” that ran in Nature. “It seems appropriate to assign the term ‘Anthropocene’ to the present, in many ways human-dominated, geological epoch,” he observed. Among the many geologic-scale changes people have effected, Crutzen cited the following:

• Human activity has transformed between a third and a half of the land surface of the planet.

• Most of the world’s major rivers have been dammed or diverted.”

“• Fertilizer plants produce more nitrogen than is fixed naturally by all terrestrial ecosystems.

• Fisheries remove more than a third of the primary production of the oceans’ coastal waters.

• Humans use more than half of the world’s readily accessible fresh water runoff.”

“Most significantly, Crutzen said, people have altered the composition of the atmosphere. Owing to a combination of fossil fuel combustion and deforestation, the concentration of carbon dioxide in the air has risen by forty percent over the last two centuries, while the concentration of methane, an even more potent greenhouse gas, has more than doubled.

“Because of these anthropogenic emissions,” Crutzen wrote, the global climate is likely to “depart significantly from natural behavior for many millennia to come.”

Crutzen published “Geology of Mankind” in 2002. Soon, the “Anthropocene” began migrating out into other scientific journals.”

“Global Analysis of River Systems: From Earth System Controls to Anthropocene Syndromes” was the title of a 2003 article in the journal Philosophical Transactions of the Royal Society B.

“Soils and Sediments in the Anthropocene” ran the headline of a piece from 2004 in the Journal of Soils and Sediments.

When Zalasiewicz came across the term, he was intrigued. He noticed that most of those using it were not trained stratigraphers, and he wondered how his colleagues felt about this. At the time, he was head of the stratigraphy committee of the Geological Society of London, the body Lyell and also William Whewell and John Phillips once presided over. At a luncheon meeting, Zalasiewicz asked his fellow committee members what they thought of the Anthropocene. Twenty-one out of the twenty-two thought that the concept had merit.”

“The group decided to examine the idea as a formal problem in geology. Would the Anthropocene satisfy the criteria used for naming a new epoch? (To geologists, an epoch is a subdivision of a period, which, in turn, is a division of an era: the Holocene, for instance, is an epoch of the Quaternary, which is a period in the Cenozoic.) The answer the members arrived at after a year’s worth of study was an unqualified “yes.” The sorts of changes that Crutzen had enumerated would, they decided, leave behind “a global stratigraphic signature” that would still be legible millions of years from now, the same way that, say, the Ordovician glaciation left behind a “stratigraphic signature” that is still legible today. Among other things, the members of the group observed in a paper summarizing their findings, the Anthropocene will be marked by a unique “biostratigraphical signal,” a product of the current extinction event on the one hand and of the human propensity for redistributing life on the other. “signal will be permanently inscribed, they wrote, “as future evolution will take place from surviving (and frequently anthropogenically relocated) stocks.” Or, as Zalasiewicz would have it, rats.”

“By the time of my visit to Scotland, Zalasiewicz had taken the case for the Anthropocene to the next level. The International Commission on Stratigraphy, or ICS, is the group responsible for maintaining the official timetable of earth’s history. It’s the ICS that settles such matters as: when exactly did the Pleistocene begin? (After much heated debate, the commission recently moved that epoch’s start date back from 1.8 to 2.6 million years ago.) Zalasiewicz had convinced the ICS to look into formally recognizing the Anthropocene, an effort that, logically enough, he himself was put in charge of. As head of the Anthropocene Working Group, Zalasiewicz is hoping to bring a proposal to a vote by the full body in 2016. If he’s successful and the Anthropocene is adopted as a new epoch, every geology textbook in the world immediately will become obsolete.”

Read TheSixth Extinction, pages 161-240.

CHAPTER VI

THE SEA AROUND US

Patella caerulea

Castello Aragonese is a tiny island that rises straight out of the Tyrrhenian Sea, like a turret. Eighteen miles west of Naples, it can be reached from the larger island of Ischia via a long, narrow stone bridge. At the end of the bridge there’s a booth where ten euros buys a ticket that allows you to climb—or, better yet, take the elevator—up to the massive castle that gives the island its name. The castle houses a display of medieval torture instruments as well as a fancy hotel and an outdoor café. On a summer evening, the café is supposed to be a pleasant place to sip Campari and contemplate the terrors of the past.

Like many small places, Castello Aragonese is a product of ” “very large forces, in this case the northward drift of Africa, which every year brings Tripoli an inch or so closer to Rome. Along a complicated set of folds, the African plate is pressing into Eurasia, the way a sheet of metal might be forced into a furnace. Occasionally, this process results in violent volcanic eruptions. (One such eruption, in 1302, led the entire population of Ischia to take refuge on Castello Aragonese.) On a more regular basis, it sends streams of gas bubbling out of vents in the sea floor. This gas, as it happens, is almost a hundred percent carbon dioxide.

Carbon dioxide has many interesting properties, one of which is that it dissolves in water to form an acid. I have come to Ischia in late January, deep into the off-season, specifically to swim in its bubbly, acidified bay. Two marine biologists, Jason Hall-Spencer and Maria Cristina Buia, have promised to show me the vents, provided the predicted rainstorm holds off. It is a raw, gray day, and we are thumping along in a fishing boat that’s been converted into a research vessel. We round Castello Aragonese and anchor about twenty yards from its “its rocky cliffs. From the boat, I can’t see the vents, but I can see signs of them. A whitish band of barnacles runs all the way around the base of the island, except above the vents, where the barnacles are missing.”

“Barnacles are pretty tough,” Hall-Spencer observes. He is British, with dirty blond hair that sticks up in unpredictable directions. He’s wearing a dry suit, which is a sort of wet suit designed to keep its owner from ever getting wet, and it makes him look as if he’s preparing for a space journey. Buia is Italian, with reddish brown hair that reaches her shoulders. She strips down to her bathing suit and pulls on her wet suit with one expert motion. I try to emulate her with a suit I have borrowed for the occasion. It is, I learn as I tug at the zipper, about half a size too small. We all put on masks and flippers and flop in.”

“The water is frigid. Hall-Spencer is carrying a knife. He pries some sea urchins from a rock and holds them out to me. Their spines are an inky black. We swim on, along the southern shore of the island, toward the vents. Hall-Spencer and Buia keep pausing to gather samples—corals, snails, seaweeds, mussels—which they place in mesh sacs that drag behind them in the water. When we get close enough, I start to see bubbles rising from the sea floor, like beads of quicksilver. Beds of seagrass wave beneath us. The blades are a peculiarly vivid green. This, I later learn, is because the tiny organisms that usually coat them, dulling their color, are missing. The closer we get to the vents, the less there is to collect. The sea urchins drop away, and so, too, do the mussels and the barnacles. Buia finds some hapless limpets attached to the cliff. Their shells have wasted away almost to the point of transparency. Swarms of jellyfish waft by, just a shade paler than the sea.

“Watch out,” Hall-Spencer warns. “They sting.”

“SINCE the start of the industrial revolution, humans have burned through enough fossil fuels—coal, oil, and natural gas—to add some 365 billion metric tons of carbon to the atmosphere. Deforestation has contributed another 180 billion tons. Each year, we throw up another nine billion tons or so, an amount that’s been increasing by as much as six percent annually. As a result of all this, the concentration of carbon dioxide in the air today—a little over four hundred parts per million—is higher than at any other point in the last eight hundred thousand years. Quite probably it is higher than at any point in the last several million years. If current trends continue, CO2 concentrations will top five hundred parts per million, roughly double the levels they were in preindustrial days, by 2050.” “It is expected that such an increase will produce an eventual average global temperature rise of between three and a half and seven degrees Fahrenheit, and this will, in turn, trigger a variety of world-altering events, including the disappearance of most remaining glaciers, the inundation of low-lying islands and coastal cities, and the melting of the Arctic ice cap. But this is only half the story.”

“Ocean covers seventy percent of the earth’s surface, and everywhere that water and air come into contact there’s an exchange. Gases from the atmosphere get absorbed by the ocean and gases dissolved in the ocean are released into the atmosphere. When the two are in equilibrium, roughly the same quantities are being dissolved as are being released. Change the atmosphere’s composition, as we have done, and the exchange becomes lopsided: more carbon dioxide enters the water than comes back out. In this way, humans are constantly adding CO2 to the seas, much as the vents do, but from above rather than below and on a global scale. This year alone the oceans will absorb two and a half billion tons of carbon, and next year it is expected they will absorb another two and a half billion tons. Every day, every American in effect pumps seven pounds of carbon into the sea.”

“Thanks to all this extra CO2, the pH of the oceans’ surface waters has already dropped, from an average of around 8.2 to an average of around 8.1. Like the Richter scale, the pH scale is logarithmic, so even such a small numerical difference represents a very large real-world change. A decline of .1 means that the oceans are now thirty percent more acidic than they were in 1800. Assuming that humans continue to burn fossil fuels, the oceans will continue to absorb carbon dioxide and will become increasingly acidified. Under what’s known as a “business as usual” emissions scenario, surface ocean pH will fall to 8.0 by the middle of this century, and it will drop to 7.8 “by the century’s end. At that point, the oceans will be 150 percent more acidic than they were at the start of the industrial revolution.*

Owing to the CO2 pouring out of the vents, the waters around Castello Aragonese provide a near-perfect preview of what lies ahead for the oceans more generally. Which is why I am paddling around the island in January, gradually growing numb from the cold. Here it is possible to swim—even, I think in a moment of panic, to drown—in the seas of tomorrow today.”

“BY the time we get back to the harbor in Ischia, the wind has come up. The deck is a clutter of spent air tanks, dripping wet suits, and chests full of samples. Once unloaded, everything has to be lugged through the narrow streets and up to the local marine biological station, which occupies a steep promontory overlooking the sea. The station was founded by a nineteenth-century German naturalist named Anton Dohrn. Hanging on the wall in the entrance hall, I notice, is a copy of a letter Charles Darwin sent to Dohrn in 1874. In it, Darwin expresses dismay at having heard, through a mutual friend, that Dohrn is overworked.”

“Installed in tanks in a basement laboratory, the animals Buia and Hall-Spencer gathered from around Castello Aragonese at first appear inert—to my untrained eye, possibly even dead. But after a while, they set about waggling their tentacles and scavenging for food. There is a starfish missing a leg, and a lump of rather rangy-looking coral, and some sea urchins, which move around their tanks on dozens of threadlike “tube feet.” (Each tube foot is controlled hydraulically, extending and retracting in response to water pressure.) There is also a six-inch-long sea cucumber, which bears an unfortunate resemblance to a blood sausage or, worse yet, a turd. In the chilly lab, the destructive effect of the vents is plain. Osilinus turbinatus is a common Mediterranean snail with a shell of alternating black and white splotches arranged in a snakeskin-like pattern. The Osilinus turbinatus in the tank has no pattern; the ridged outer layer of its shell has been eaten away, exposing the smooth, all-white layer underneath. The limpet Patella caerulea is shaped like a Chinese straw hat. Several Patella caerulea shells have deep lesions through which their owners’ putty-colored bodies can be seen. They look as if they have been dunked in acid, which in a manner of speaking they have.

“Because it’s so important, we humans put a lot of energy into making sure that the pH of our blood is constant,” Hall-Spencer says, raising his voice to be heard over the noise of the running water. “But some of these lower organisms, they don’t have the physiology to do that. They’ve just got to tolerate what’s happening outside, and so they get pushed beyond their limits.”

“Later, over pizza, Hall-Spencer tells me about his first trip to the vents. That was in the summer of 2002, when he was working on an Italian research vessel called the Urania. One hot day, the Urania was passing by Ischia when the crew decided to anchor and go for a swim. Some of the Italian scientists who knew about the vents took Hall-Spencer to see them, just for the fun of it. He enjoyed the novelty of the experience—swimming through the bubbles is a bit like bathing in champagne—but beyond that, it set him thinking.

At the time, marine biologists were just beginning to recognize the hazards posed by acidification. Some disturbing calculations had been done and some preliminary experiments performed on animals raised in labs. It occurred to Hall-Spencer that the vents could be used for a new and more ambitious sort of study. This one would involve not just a few species reared in tanks, but dozens of species living and breeding in their natural (or, if you prefer, naturally unnatural) environment.”

“At Castello Aragonese, the vents produce a pH gradient. On the eastern edge of the island, the waters are more or less unaffected. This zone might be thought of as the Mediterranean of the present. As you move closer to the vents, the acidity of the water increases and the pH declines. A map of life along this pH gradient, Hall-Spencer reasoned, would represent a map of what lies ahead for the world’s oceans. It would be like having access to an underwater time machine.

It took Hall-Spencer two years to get back to Ischia. He did not yet have funding for his project, and so he had trouble getting anyone to take him seriously. Unable to afford a hotel room, he camped out on a ledge in the cliffs. To collect samples, he used discarded plastic water bottles. “It was a bit Robinson Crusoe-ish,” he tells me.”

“Eventually, he convinced enough people, including Buia, that he was onto something. Their first task was producing a detailed survey of pH levels around the island. Then they organized a census of what was living in each of the different pH zones. This involved placing metal frames along the shore and registering every mussel, barnacle, “and limpet clinging to the rocks. It also involved spending hours at a stretch sitting underwater, counting passing fish.”

“In the waters far from the vents Hall-Spencer and his colleagues found a fairly typical assemblage of Mediterranean species. These included: Agelas oroides, a sponge that looks a bit like foam insulation; Sarpa salpa, a commonly consumed fish that, on occasion, causes hallucinations; and Arbacia lixula, a sea urchin with a lilac tinge. Also living in the area was Amphiroa rigida, a spiky, pinkish seaweed, and Halimeda tuna, a green seaweed that grows in the shape of a series of connecting disks. (The census was limited to creatures large enough to be seen with the naked eye.) In this vent-free zone, sixty-nine species of animals and fifty-one species of plants were counted.”

“When Hall-Spencer and his team set up their quadrants closer to the vents, the tally they came up with was very different. Balanus perforatus is a grayish barnacle that resembles a tiny volcano. It is common and abundant from west Africa to Wales. In the pH 7.8 zone, which corresponds to the seas of the not-too-distant future, Balanus perforatus was gone. Mytilus galloprovincialis, a blue-black mussel native to the Mediterranean, is so adaptable that it’s established itself in many parts of the world as an invasive. It, too, was missing. Also absent were: Corallina elongata and Corallina officinalis, both forms of stiff, reddish seaweed; Pomatoceros triqueter, a kind of keel worm; three “species of coral; several species of snails; and Arca noae, a mollusk commonly known as Noah’s Ark. All told, one-third of the species found in the vent-free zone were no-shows in the pH 7.8 zone.

“Unfortunately, the biggest tipping point, the one at which the ecosystem starts to crash, is mean pH 7.8, which is what we’re expecting to happen by 2100,” Hall-Spencer tells me, in his understated British manner. “So that is rather alarming.”

“SINCE Hall-Spencer’s first paper on the vent system appeared, in 2008, there has been an explosion of interest in acidification and its effects. International research projects with names like BIOACID (Biological Impacts of Ocean Acidification) and EPOCA (the European Project on Ocean Acidification) have been funded, and hundreds, perhaps thousands, of experiments have been undertaken. These experiments have been conducted on board ships, in laboratories, and in enclosures known as mesocosms, which allow conditions to be manipulated on a patch of actual ocean.

Again and again, these experiments have confirmed the hazards posed by rising CO2. While many species will apparently do fine, even thrive in an acidified ocean, lots of others will not. Some of the organisms that have been shown to be vulnerable, like clownfish and Pacific oysters, are familiar from aquariums ”

“and the dinner table; others are less charismatic (or tasty) but probably more essential to marine ecosystems. Emiliania huxleyi, for example, is a single-celled phytoplankton—a coccolithophore—that surrounds itself with tiny calcite plates. Under magnification, it looks like some kind of crazy crafts project: a soccer ball covered in buttons. It is so common at certain times of year that it turns vast sections of the seas a milky white, and it forms the base of many marine food chains. Limacina helicina is a species of pteropod, or “sea butterfly,” that resembles a winged snail. It lives in the Arctic and is an important food source for many much larger animals, including herring, salmon, and whales. Both of these species appear to be highly sensitive to acidification: in one mesocosm experiment Emiliania huxleyi disappeared altogether from enclosures with elevated CO2 levels.”

“Ulf Riebesell is a biological oceanographer at the GEOMAR-Helmholtz Centre for Ocean Research in Kiel, Germany, who has directed several major ocean acidification studies, off the coasts of Norway, Finland, and Svalbard. Riebesell has found that the groups that tend to fare best in acidified water are plankton that are so tiny—less than two microns across—that they form their own microscopic food web. As their numbers “increase, these picoplankton, as they are called, use up more nutrients, and larger organisms suffer.

“If you ask me what’s going to happen in the future, I think the strongest evidence we have is there is going to be a reduction in biodiversity,” Riebesell told me. “Some highly tolerant organisms will become more abundant, but overall diversity will be lost. This is what has happened in all these times of major mass extinction.”

“Ocean acidification is sometimes referred to as global warming’s “equally evil twin.” The irony is intentional and fair enough as far as it goes, which may not be far enough. No single mechanism explains all the mass extinctions in the record, and yet changes in ocean chemistry seem to be a pretty good predictor. Ocean acidification played a role in at least two of the Big Five extinctions (the end-Permian and the end-Triassic) and quite possibly it was a major factor in a third (the end-Cretaceous). There’s strong evidence for ocean acidification during an extinction event known as the Toarcian Turnover, which occurred 183 million years ago, in the early Jurassic, and similar evidence at the end of the Paleocene, 55 million years ago, when several forms of marine life suffered a major crisis.”

“Oh, ocean acidification,” Zalasiewicz had told me at Dob’s Linn. “That’s the big nasty one that’s coming down.”

*   *   *

WHY is ocean acidification so dangerous? The question is tough to answer only because the list of reasons is so long. Depending on how tightly organisms are able to regulate their internal chemistry, acidification may affect such basic processes as metabolism, enzyme activity, and protein function. Because it will change the makeup of microbial communities, it will alter the availability of key nutrients, like iron and nitrogen. For similar reasons, it will change the amount of light that passes through the water, and for somewhat different reasons, it will alter the way sound propagates. (In general, acidification is expected to make the seas noisier.) It seems likely to promote the growth of toxic algae. It will impact photosynthesis—many plant species are apt to benefit from elevated CO2 levels—and it will alter the compounds formed by dissolved metals, in some cases in ways that could be poisonous.”

“Of the myriad possible impacts, probably the most significant involves the group of creatures known as calcifiers. (The term calcifier applies to any organism that builds a shell or external skeleton or, in the case of plants, a kind of internal scaffolding out of the mineral calcium carbonate.) Marine calcifiers are a fantastically varied lot. Echinoderms like starfish and sea urchins are calcifiers, as are mollusks like clams and oysters. So, too, are barnacles, which are crustaceans. Many species of coral are calcifiers; this is how they construct the towering structures that become reefs. Lots of kinds of seaweed are calcifiers; these often feel rigid or brittle to the touch. Coralline algae—minute organisms that grow in colonies that look like a smear of pink paint—are also calcifiers. Brachiopods are calcifiers, and so are coccolithophores, foraminifera, and many types of pteropods—the list goes on and on. It’s been estimated that calcification evolved at least two dozen separate times over the course of life’s history, and it’s quite possible that the number is higher than that.”

“From a human perspective, calcification looks a bit like construction work and also a bit like alchemy. To build their shells or exoskeletons or calcitic plates, calcifiers must join calcium ions (Ca2+) and carbonate ions (CO32−) to form calcium carbonate (CaCO3). But at the concentrations that they’re found in ordinary seawater, calcium and carbonate ions won’t combine. At the site of calcification, organisms must therefore alter the chemistry of the water to, in effect, impose a chemistry of their own.”

“Ocean acidification increases the cost of calcification by reducing the number of carbonate ions available to begin with. To extend the construction metaphor, imagine trying to build a house while someone keeps stealing your bricks. The more acidified the water, the greater the energy that’s required to complete the necessary steps. At a certain point, the water becomes positively corrosive and solid calcium carbonate begins to dissolve. This is why the limpets that wander too close to the vents at Castello Aragonese end up with holes in their shells.”

“Lab experiments have indicated that calcifiers will be particularly hard-hit by falling ocean pH, and the list of the disappeared at Castello Aragonese confirms this. In the pH 7.8 zone, three-quarters of the missing species are calcifiers. These include the nearly ubiquitous barnacle Balanus perforatus, the hardy mussel Mytilus galloprovincialis, and the keel worm Pomatoceros triqueter. Other absent calcifiers are Lima lima, a common bivalve; Jujubinus striatus, a chocolate-colored sea snail; and Serpulorbis arenarius, a mollusk known as a worm snail. Calcifying seaweed, meanwhile, is completely absent.”

“According to geologists who work in the area, the vents at Castello Aragonese have been spewing carbon dioxide for at least several hundred years, maybe longer. Any mussel or barnacle or keel worm that can adapt to lower pH in a time frame of centuries presumably already would have done so. “You give them generations on generations to survive in these conditions, and yet they’re not there,” Hall-Spencer observed.”

“And the lower the pH drops, the worse it goes for calcifiers. Right up near the vents, where the bubbles of CO2 stream up in thick ribbons, Hall-Spencer found that they are entirely absent. In fact, all that remains in this area—the underwater equivalent of a vacant lot—are a few hardy species of native algae, some “species of invasive algae, one kind of shrimp, a sponge, and two kinds of sea slugs.

“You won’t see any calcifying organisms, full stop, in the area where the bubbles are coming up,” he told me. “You know how normally in a polluted harbor you’ve got just a few species that are weedlike and able to cope with massively fluctuating conditions? Well, it’s like that when you ramp up CO2.”

“ROUGHLY one-third of the CO2 that humans have so far pumped into the air has been absorbed by the oceans. This comes to a stunning 150 billion metric tons. As with most aspects of the Anthropocene, though, it’s not only the scale of the transfer but also the speed that’s significant. A useful (though admittedly imperfect) comparison can be made to alcohol. Just as it makes a big difference to your blood chemistry whether you take a month to go through a six-pack or an hour, it makes a big difference to marine chemistry whether carbon dioxide is added over the course of a million years or a hundred. To the oceans, as to the human liver, rate matters.”

“If we were adding CO2 to the air more slowly, geophysical processes, like the weathering of rock, would come into play to counteract acidification. As it is, things are moving too fast for such slow-acting forces to keep up. As Rachel Carson once observed, referring to a very different but at the same time profoundly similar problem: “Time is the essential ingredient, but in the modern world there is no time.”

“A group of scientists led by Bärbel Hönisch, of Columbia’s Lamont-Doherty Earth Observatory, recently reviewed the evidence for changing CO2 levels in the geologic past and concluded that, although there are several severe episodes of ocean acidification in the record, “no past event perfectly parallels” what is happening right now, owing to “the unprecedented rapidity of CO2 release currently taking place.” It turns out there just aren’t many ways to inject billions of tons of carbon into the air very quickly. The best explanation anyone has come up with for the end-Permian extinction is a massive burst of vulcanism in what’s now Siberia. But even this spectacular event, which created the formation known as the Siberian Traps, probably released, on an annual basis, less carbon than our cars and factories and power plants.”

“By burning through coal and oil deposits, humans are putting carbon back into the air that has been sequestered for tens—in most cases hundreds—of millions of years. In the process, we are running geologic history not only in reverse but at warp speed.

“It is the rate of CO2 release that makes the current great experiment so geologically unusual, and quite probably unprecedented in earth history,” Lee Kump, a geologist at Penn State, and Andy Ridgwell, a climate modeler from the University of “Bristol, observed in a special issue of the journal Oceanography devoted to acidification. Continuing along this path for much longer, the pair continued, “is likely to leave a legacy of the Anthropocene as one of the most notable, if not cataclysmic events in the history of our planet.”

“CHAPTER VII

DROPPING ACID

Acropora millepora”

Excerpt From:

Elizabeth Kolbert. “The Sixth Extinction.

” iBooks. https://itunes.apple.com/us/book/the-sixth-extinction/id687060053?mt=11

“Half a world away from Castello Aragonese, One Tree Island sits at the southernmost tip of the Great Barrier Reef, about fifty miles off the coast of Australia. It has more than one tree, which surprised me when I got there, expecting—cartoonishly, I suppose—a single palm sticking up out of white sand. As it turned out, there wasn’t any sand, either. The whole island consists of pieces of coral rubble, ranging in size from small marbles to huge boulders. Like the living corals they once were part of, the rubble chunks come in dozens of forms. Some are stubby and finger-shaped, others branching, like a candelabra. Still others resemble antlers or dinner plates or bits of brain. It is believed that One Tree Island was created during a particularly vicious storm that occurred some four thousand years ago. (As one geologist who has studied the place put it to me, “You wouldn’t have wanted to be there when that happened.”) The island is still in the process of changing shape; a storm that passed through in March 2009—Cyclone Hamish—added a ridge that runs along the island’s eastern shore.”

“One Tree would qualify as deserted except for a tiny research station operated by the University of Sydney. I traveled to the island, as just about everyone does, from another, slightly larger island about twelve miles away. (That island is known as Heron Island, also a misnomer, since at Heron there are no herons.) When we docked—or really moored, since One Tree has no dock—a loggerhead turtle was heaving herself out of the water onto the shore. She was nearly four feet long, with a large welt on her shell, which was encrusted with ancient-looking barnacles. News travels fast on a nearly deserted island, and soon the entire human population of One Tree—twelve people, including me—had come out to watch. “Sea turtles usually lay their eggs at night, on sandy beaches; this was in the middle of the day, on jagged coral rubble. The turtle tried to dig a hole with her back flippers. After much exertion, she produced a shallow trough. By this point, one of her flippers was bleeding. She heaved herself farther up the shore and tried again, with similar results. She was still at it an hour and a half later, when I had to go get a safety lecture from the manager of the research station, Russell Graham. “He warned me not to go swimming when the tide was going out, as I might find myself “swept off to Fiji.” (This was a line I would hear repeated many times during my stay, though there was some disagreement about whether the current was heading toward Fiji or really away from it.) Once I’d taken in this and other advisories—the bite of a blue-ringed octopus is usually fatal; the sting of a stonefish is not, but it is so painful it will make you wish it were—I went back to see how the turtle was doing. Apparently, she had given up and crawled back into the sea.”

“The One Tree Island Research Station is a bare-bones affair. It consists of two makeshift labs, a pair of cabins, and an outhouse with a composting toilet. The cabins rest directly on the rubble, for the most part with no floor, so that even when you’re indoors you feel as if you’re out. Teams of scientists from all around the world book themselves into the station for stays of a few weeks or a few months. At one point, someone must have decided that every team should leave a record of its visit on the cabin walls. GETTING TO THE CORE IN 2004, reads one inscription, drawn in magic marker. Others include:

THE CRAB CREW: CLAWS FOR A CAUSE—2005

CORAL SEX—2008

THE FLUORESCENCE TEAM—2009”

“The American-Israeli team that was in residence at the time of my arrival had already made two trips to the island. The epigram from its first visit, DROPPING ACID ON CORALS, was accompanied by a sketch of a syringe dripping what looked like blood onto a globe. The group’s latest message referred to its study site, a patch of coral known as DK-13. DK-13 lies out on the reef, far enough away from the station that, for the purposes of communication, it might as well be on the moon.”

“The writing on the wall said, DK-13: NO ONE CAN HEAR YOU SCREAM.

*   *   *

THE first European to encounter the Great Barrier Reef was Captain James Cook. In the spring of 1770, Cook was sailing along the east coast of Australia when his ship, the Endeavour, rammed into a section of the reef about thirty miles southeast of what is now, not coincidentally, Cooktown. Everything dispensable, including the ship’s cannon, was tossed overboard, and the leaky Endeavour managed to creak ashore, where the crew spent the next two months repairing its hull. “Cook was flummoxed by what he described as “a wall of Coral Rock rising all most perpendicular out of the unfathomable Ocean.” He understood that the reef was biological in origin, that it had been “formed in the Sea by animals.” But how, then, he would later ask, had it come to be “thrown up to such a height?”

The question of how coral reefs arose was still an open one sixty years later, when Lyell sat down to write the Principles. Although he had never seen a reef, Lyell was fascinated by them, and he devoted part of volume two to speculating about their origins. Lyell’s theory—that reefs grew from the rims of extinct underwater volcanoes—he borrowed more or less wholesale from a Russian naturalist named Johann Friedrich von Eschscholtz. (Before Bikini Atoll became Bikini Atoll, it was called, rather less enticingly, Eschsholtz Atoll.)”

“When his turn came to theorize about reefs, Darwin had the advantage of actually having visited some. In November 1835, the Beagle moored off Tahiti. Darwin climbed to one of the highest points on the island, and from there he could survey the neighboring island of Moorea. Moorea, he observed, was encircled by a reef the way a framed etching is surrounded by a mat.”

“I am glad that we have visited these islands,” Darwin wrote in his diary, for coral reefs “rank high amongst the wonderful objects in the world.” Looking over at Moorea and its surrounding reef, he pictured time running forward; if the island were to sink away, Moorea’s reef would become an atoll. When Darwin returned to London and shared his subsidence theory with Lyell, Lyell, though impressed, foresaw resistance. “Do not flatter yourself that you will be believed until you are growing bald like me,” he warned.”

“In fact, debate about Darwin’s theory—the subject of his 1842 book The Structure and Distribution of Coral Reefs—continued until the nineteen-fifties, when the U.S. Navy arrived in the Marshall Islands with plans to vaporize some of them. In preparation for the H-bomb tests, the Navy drilled a series of cores on an atoll called Enewetak. As one of Darwin’s biographers put it, these cores proved his theory to be, in its large lines at least, “astoundingly correct.”

“Darwin’s description of coral reefs as “amongst the wonderful objects of the world” also still stands. Indeed, the more that has been learned about reefs, the more marvelous they seem. Reefs are organic paradoxes—obdurate, ship-destroying ramparts constructed by tiny gelatinous creatures. They are part animal, part vegetable, and part mineral, at once teeming with life and, at the same time, mostly dead.”

“Like sea urchins and starfish and clams and oysters and barnacles, reef-building corals have mastered the alchemy of calcification. What sets them apart from other calcifiers is that instead of working solo, to produce a shell, say, or some calcitic plates, corals engage in vast communal building projects that stretch over generations. Each individual, known unflatteringly as a polyp, adds to its colony’s collective exoskeleton. On a reef, billions of polyps belonging to as many as a hundred different species “are all devoting themselves to this same basic task. Given enough time (and the right conditions), the result is another paradox: a living structure. The Great Barrier Reef extends, discontinuously, for more than fifteen hundred miles, and in some places it is five hundred feet thick. By the scale of reefs, the pyramids at Giza are kiddie blocks.”

“The way corals change the world—with huge construction projects spanning multiple generations—might be likened to the way that humans do, with this crucial difference. Instead of displacing other creatures, corals support them. Thousands—perhaps millions—of species have evolved to rely on coral reefs, either directly for protection or food, or indirectly, to prey on those species that come seeking protection or food. “This coevolutionary venture has been under way for many geologic epochs. Researchers now believe it won’t last out the Anthropocene. “It is likely that reefs will be the first major ecosystem in the modern era to become ecologically extinct” is how a trio of British scientists recently put it. Some give reefs until the end of the century, others less time even than that. A paper published in Nature by the former head of the One Tree Island Research Station, Ove Hoegh-Guldberg, predicted that if current trends continue, then by around 2050 visitors to the Great Barrier Reef will arrive to find “rapidly eroding rubble banks.”

“ CAME to One Tree more or less by accident. My original plan had been to stay on Heron Island, where there’s a much larger research station and also a ritzy resort. On Heron, I was going to watch the annual coral spawning and observe what had been described to me in various Skype conversations as a seminal experiment on ocean acidification. Researchers from the University of Queensland were building an elaborate Plexiglas mesocosm that was going to allow them to manipulate CO2 levels on a patch of reef, even as it allowed the various creatures that depend on the reef to swim in and out. By changing the pH inside the mesocosm and measuring what happened to the corals, they were going to be able to generate predictions about the reef as a whole. I arrived at Heron in time to see the spawning—more on this later—but the experiment was way behind schedule and the mesocosm still in pieces. Instead of the reef of the future, all there was to see was a bunch of anxious graduate students hunched over soldering irons in the lab.”

“As I was trying to figure out what to do next, I heard about another experiment on corals and ocean acidification that was under way at One Tree, which, by the scale of the Great Barrier Reef, lies just around the corner. Three days later—there is no regular transportation to One Tree—I managed to get a boat over.

The head of the team at One Tree was an atmospheric scientist named Ken Caldeira. Caldeira, who’s based at Stanford, is “often credited with having coined the term “ocean acidification.” He became interested in the subject in the late nineteen-nineties when he was hired to do a project for the Department of Energy. The department wanted to know what the consequences would be of capturing carbon dioxide from smokestacks and injecting it into the deep sea. At that point, almost no modeling work had been done on the effects of carbon emissions on the oceans. Caldeira set about calculating how the ocean’s pH would change as a result of deep-sea injection, and then compared that result with the current practice of pumping CO2 into the atmosphere and allowing it to be absorbed by surface waters. In 2003, he submitted his results to Nature. The journal’s editors advised him to drop the discussion of deep-ocean injection because the calculations concerning the effects of ordinary atmospheric release were so startling. Caldeira published the first part of his paper under the subheading “The Coming Centuries May See More Ocean Acidification Than the Past 300 Million Years.”

“Under business as usual, by mid-century things are looking rather grim,” he told me a few hours after I had arrived at One Tree. We were sitting at a beat-up picnic table, looking out over the heartbreaking blue of the Coral Sea. The island’s large and boisterous population of terns was screaming in the background. Caldeira paused: “I mean, they’re looking grim already.”

“CALDEIRA, who is in his mid-fifties, has curly brown hair, a boyish smile, and a voice that tends to rise toward the end of sentences, so that it often seems he is posing a question even when he’s not. Before getting into research, he worked as a software developer on Wall Street. One of his clients was the New York Stock Exchange, for whom he designed a computer program to detect insider trading. The program functioned as it was supposed to, but after a while Caldeira came to believe that the NYSE wasn’t really interested in catching insider traders, and he decided to switch professions.”

“Unlike most atmospheric scientists, who focus on one particular aspect of the system, Caldeira is, at any given moment, working on four or five disparate projects. He particularly likes computations of a provocative or surprising nature; for example, he once calculated that cutting down all the world’s forests and replacing them with grasslands would have a slight cooling effect. (Grasslands, which are lighter in color than forests, absorb less sunlight.) Other calculations of his show that to keep pace with the present rate of temperature change, plants and animals would have to migrate poleward by thirty feet a day, and that a molecule of CO2 generated by burning fossil fuels will, in the course of its lifetime in the atmosphere, trap a hundred thousand times more heat than was released in producing it.”

“At One Tree, life for Caldeira and his team revolved around the tides. An hour before the first low tide of the day and then an hour afterward, someone had to collect water samples out at DK-13, so named because the Australian researcher who had set up the site, Donald Kinsey, had labeled it with his initials. A little more than twelve hours later, the process would be repeated, and so on, from one low tide to the next. The experiment was slow tech rather than high tech; the idea was to measure various properties of the water that Kinsey had measured back in the nineteen-seventies, then compare the two sets of data and try to tease out how calcification rates on the reef had changed in the intervening decades. In daylight, the trip to DK-13 could be made by one person. In the dark, in deference to the fact that “no one can hear you scream,” the rule was that two had to go.

My first evening on One Tree, low tide fell at 8:53 PM. Caldeira was making the post–low-tide trip, and I volunteered to go with him. At around nine “o’clock, we gathered up half a dozen sampling bottles, a pair of flashlights, and a handheld GPS unit and started out.”

“From the research station, it was about a mile walk to DK-13. The route, which someone had plugged into the GPS unit, led around the southern tip of the island and over a slick expanse of rubble that had been nicknamed the “algal highway.” From there it veered out onto the reef itself.”

“Since corals like light but can’t survive long exposure to the air, they tend to grow as high as the water level at low tide and then spread out laterally. This produces an expanse of reef that’s more or less flat, like a series of tables, which can be crossed the way a kid, after school, might jump from desk to desk. The surface of One Tree’s reef flat was brittle and brownish and was known around the research station as the “pie crust.” It crackled ominously underfoot. Caldeira warned me that if I fell through, it would be bad for the reef and even worse for my shins. I recalled another message I had seen penned on the wall of the research station: DON’T TRUST THE PIE CRUST.”

“The night was balmy and, beyond the beams of our flashlights, pitch-black. Even in the dark, the extraordinary vitality of the reef was evident. We passed several loggerhead turtles waiting out low tide with what looked like bored expressions. We encountered bright blue starfish, and leopard sharks stranded in shallow pools, and ruddy octopuses doing their best to blend into the reef. Every few feet, we had to step over a giant clam, which appeared to be leering with garishly painted lips. (The mantles of giant clams are packed with colorful symbiotic algae.) The sandy strips between the blocks of coral were littered with sea cucumbers, which, despite the name, are animals whose closest relations are sea urchins. On the Great Barrier Reef, the sea cucumbers are the size not of cucumbers but of bolster cushions. Out of curiosity, I decided to pick one up. It “was about two feet long and inky black. It felt like slime-covered velvet.”

“After a few wrong turns and several delays while Caldeira tried to photograph the octopuses with a waterproof camera, we reached DK-13. The site consisted of nothing more than a yellow buoy and some sensing equipment anchored to the reef with a rope. I glanced back in what I thought was the direction of the island, but there was no island, or land of any sort, to be seen. We rinsed out the sampling bottles, filled them, and started back. The darkness was, if anything, even more complete. The stars were so bright they appeared to be straining out of the sky. For a brief moment I felt I understood what it must have been like for an explorer like Cook to arrive at such a place, at the edge of the known world.”

“CORAL reefs grow in a great swath that stretches like a belt around the belly of the earth, from thirty degrees north to thirty degrees south latitude. After the Great Barrier Reef, the world’s second-largest reef is off the coast of Belize. There are extensive coral reefs in the tropical Pacific, in the Indian Ocean, and in the Red Sea, and many smaller ones in the Caribbean. Yet curiously enough, the first evidence that CO2 could kill a reef came from Arizona, from the self-enclosed, supposedly self-sufficient world known as Biosphere 2.”

“A three-acre, glassed-in structure shaped like a ziggurat, Biosphere 2 was built in the late nineteen-eighties by a private group largely funded by the billionaire Edward Bass. It was intended to demonstrate how life on earth—Biosphere 1—could be re-created on, say, Mars. The building contained a “rainforest,” a “desert,” an “agricultural zone,” and an artificial “ocean.” The first group of Biospherians, four men and four women, remained sealed inside the place for two years. They grew all of their own food and, for a stretch, breathed only recycled air. Still, the project was widely considered a failure. The Biospherians spent much of their time hungry, and, even more ominously, they lost control of their artificial atmosphere. “In the various “ecosystems,” decomposition, which takes up oxygen and gives off carbon dioxide, was supposed to be balanced by photosynthesis, which does the reverse. For reasons mainly having to do with the richness of the soil that had been imported into the “agricultural zone,” decomposition won out. Oxygen levels inside the building fell sharply, and the Biospherians developed what amounted to altitude sickness. Carbon dioxide levels, meanwhile, soared. Eventually, they reached three thousand parts per million, roughly eight times the levels outside.”

“Biosphere 2 officially collapsed in 1995, and Columbia University took over the management of the building. The “ocean,” a tank the size of an Olympic swimming pool, was by this point a wreck: most of the fish it had been stocked with were dead, “and the corals were just barely hanging on. A marine biologist named Chris Langdon was assigned the task of figuring out something educational to do with the tank. His first step was to adjust the water chemistry. Not surprisingly, given the high CO2 content of the air, the pH of the “ocean” was low. Langdon tried to fix this, but strange things kept happening. Figuring out why became something of an obsession. After a while, Langdon sold his house in New York and moved to Arizona, so that he could experiment on the “ocean” full-time.

“Although the effects of acidification are generally expressed in terms of pH, there’s another way to look at what’s going on that’s just as important—to many organisms probably more important—and this is in terms of a property of seawater known, rather cumbersomely, as the “saturation state with respect to calcium carbonate,” or, alternatively, the “saturation state with respect to aragonite.” (Calcium carbonate comes in two different forms, depending on its crystal structure; aragonite, which is the form corals manufacture, is the more soluble variety.) The saturation state is determined by a complicated chemical formula; essentially, it’s a measure of the concentration of calcium and carbonate ions floating around. When CO2 dissolves in water, it forms carbonic acid—H2CO3—which effectively “eats” carbonate ions, thus lowering the saturation state.”

“When Langdon showed up at Biosphere 2, the prevailing view among marine biologists was that corals did not much care about the saturation state as long as it remained above one. (Below one, water is “undersaturated,” and calcium carbonate dissolves.) Based on what he was seeing, Langdon became convinced that corals did care about the saturation state; indeed, they cared about it deeply. “To test his hypothesis, Langdon employed a straightforward, if time-consuming, procedure. Conditions in the “ocean” would be varied, and small colonies of corals, which were attached to little tiles, would be periodically lifted out of the water and weighed. If the colony was putting on weight, it would show that it was growing—adding more mass through calcification. The experiment took more than three years to complete and yielded more than a thousand measurements. It revealed a more or less linear relationship between the growth rate of the corals and the saturation state of the water. Corals grew fastest at an aragonite saturation state of five, slower at four, and still slower at three. At a level of two, they basically quit building, like frustrated contractors throwing up their hands. In the artificial world of Biosphere 2, the implications of this discovery were interesting. In the real world—Biosphere 1—they were rather more worrisome.”

“Prior to the industrial revolution, all of the world’s major reefs could be found in water with an aragonite saturation state between four and five. Today, there’s almost no place left on the planet where the saturation state is above four, and if current emissions trends continue, by 2060 there will be no regions left “above 3.5. By 2100, none will remain above three. As saturation levels fall, the energy required for calcification will increase, and calcification rates will decline. Eventually, saturation levels may drop so low that corals quit calcifying altogether, but long before that point, they will be in trouble. This is because out in the real world, reefs are constantly being eaten away at by fish and sea urchins and burrowing worms. They are also being battered by waves and storms, like the one that created One Tree. Thus, just to hold their own, reefs must always be growing.

“It’s like a tree with bugs,” Langdon once told me. “It needs to grow pretty quickly just to stay even.”

“Langdon published his results in 2000. At that point many marine biologists were skeptical, in no small part, it seems, because of his association with the discredited Biosphere project. Langdon spent another two years redoing his experiments, this time with even tighter controls. The findings were the same. In the meantime, other researchers launched their own studies. These, too, confirmed Langdon’s discovery: reef-building corals are sensitive to the saturation state. This has now been shown in dozens more lab studies and also on an actual reef. A few years ago, Langdon and some colleagues conducted an experiment on a stretch of reef near a volcanic vent system off Papua New Guinea. The experiment, modeled on Hall-Spencer’s work at Castello Aragonese, again used the volcanic vents as a natural source of acidification. “As the saturation state of the water dropped, coral diversity plunged. Coralline algae declined even more drastically, an ominous sign since coralline algae act like a kind of reef glue, cementing the structure together. Seagrass, meanwhile, thrived.

“A few decades ago I, myself, would have thought it ridiculous to imagine that reefs might have a limited lifespan,” J. E. N. Veron, former chief scientist of the Australian Institute of Marine Science, has written. “Yet here I am today, humbled to have spent the most productive scientific years of my life around the rich wonders of the underwater world, and utterly convinced that they will not be there for our children’s children to enjoy.” A recent study by a team of Australian researchers found that coral cover in the Great Barrier Reef has declined by fifty percent just in the last thirty years.

Not long before their trip to One Tree, Caldeira and some of the other members of his team published a paper assessing the future of corals, using both computer models and data gathered in the field. The paper concluded that if current emissions trends continue, within the next fifty years or so “all coral reefs will cease to grow and start to dissolve.”

“IN between trips out over the reef to collect samples, the scientists at One Tree did a lot of snorkeling. The group’s preferred spot was about a half a mile offshore, on the opposite side of the island from DK-13, and getting there meant cajoling Graham, the station manager, into taking out the boat, something that he did only with reluctance and a fair amount of grumbling.”

“Some of the scientists, who had dived all over—in the Philippines, in Indonesia, in the Caribbean, and in the South Pacific—told me that the snorkeling at One Tree was about as good as it gets. I found this easy to believe. The first time I jumped off the boat and looked down at the swirl of life beneath me, it felt unreal, as if I’d swum into the undersea world of Jacques Cousteau. Schools of small fish were followed by schools of larger fish, which were followed by sharks. Huge rays glided by, trailed by turtles the size of bathtubs. I tried to keep a mental list of what I’d seen, but it was like trying to catalog a dream. After each outing, I spent hours looking through a huge volume called The Fishes of the Great Barrier Reef and the Coral Sea. “Among the fish that I think I may have spotted were: tiger sharks, lemon sharks, gray reef sharks, blue-spine unicorn fish, yellow boxfish, spotted boxfish, conspicuous angelfish, Barrier Reef anemonefish, Barrier Reef chromis, minifin parrotfish, Pacific longnose parrotfish, somber sweetlips, fourspot herring, yellowfin tuna, common dolphinfish, deceiver fangblenny, yellow spotted sawtail, barred rabbitfish, blunt-headed wrasse, and striped cleaner wrasse.”

“Reefs are often compared to rainforests, and in terms of the sheer variety of life, the comparison is apt. Choose just about any group you like, and the numbers are staggering. An Australian researcher once broke apart a volleyball-sized chunk of coral and found, living inside of it, more than fourteen hundred polychaete worms belonging to 103 different species. More recently, American researchers cracked open chunks of corals to look for crustaceans; in a square meter’s worth collected near Heron Island, they found representatives of more than a hundred species, and in a similar-sized sample, collected at the northern tip of the Great Barrier Reef, they found representatives of more than a hundred and twenty. It is estimated that at least half a million and possibly as many as nine million species spend at least part of their lives on coral reefs.”

“This diversity is all the more astonishing in light of the underlying conditions. Tropical waters tend to be low in nutrients, like nitrogen and phosphorus, which are crucial to most forms of life. (This has to do with what’s called the thermal structure of the water column, and it’s why tropical waters are often so beautifully clear.) As a consequence, the seas in the tropics should be barren—the aqueous equivalent of deserts. Reefs are thus not just underwater rainforests; they are rainforests in a marine Sahara. The first person to be perplexed by this incongruity was Darwin, and it has since become known as “Darwin’s paradox.” Darwin’s paradox has never been entirely resolved, but one key to the puzzle seems to be recycling. Reefs—or, really, reef creatures—have developed a fantastically efficient system by which nutrients are passed from one class of organisms to another, as at a giant bazaar. Corals are the main players in this complex system of exchange, and, at the same time, they provide the platform that makes the trading possible. Without them, there’s just more watery desert.

“Corals build the architecture of the ecosystem,” Caldeira told me. “So it’s pretty clear if they go, the whole ecosystem goes.”

“One of the Israeli scientists, Jack Silverman, put it to me this way: “If you don’t have a building, where are the tenants going to go?”

“REEFS have come and gone several times in the past, and their remains crop up in all sorts of unlikely places. The ruins of reefs from the Triassic, for example, can now be found towering thousands of feet above sea level in the Austrian Alps. The Guadalupe Mountains in west Texas are what’s left of reefs from the Permian period that were elevated in an episode of “tectonic compression” about eighty million years ago. Reefs from the Silurian period can be seen in northern Greenland.”

“All these ancient reefs consist of limestone, but the creatures that created them were quite different. Among the organisms that built reefs in the Cretaceous were enormous bivalves known as rudists. In the Silurian, reef builders included spongelike creatures called stromatoporoids, or “stroms” for short. In the Devonian, reefs were constructed by rugose corals, which grew in the shape of horns, and tabulate corals, which grew in the shape of honeycombs. Both rugose corals and tabulate corals were only distantly related to today’s scleractinian corals, and both orders died out in the great extinction at the end of the Permian. This extinction shows up in the geologic record as (among other things) a “reef gap”—a period of about ten million years when reefs went missing altogether.”

“Reef gaps also occurred after the late Devonian and the late Triassic extinctions, and in each of these cases it also took millions of years for reef construction to resume. This correlation has prompted some scientists to argue that reef building as an enterprise must be particularly vulnerable to environmental change—yet another paradox, since reef building is also one of the oldest enterprises on earth.”

“Ocean acidification is, of course, not the only threat reefs are under. Indeed, in some parts of the world, reefs probably will not last long enough for ocean acidification to finish them off. The roster of perils includes, but is not limited to: overfishing, which promotes the growth of algae that compete with corals; agricultural runoff, which also encourages algae growth; deforestation, which leads to siltation and reduces water clarity; and dynamite fishing, whose destructive potential would seem to be self-explanatory. All of these stresses make corals susceptible to ”

“pathogens. White-band disease is a bacterial infection that, as the name suggests, produces a band of white necrotic tissue. It afflicts two species of Caribbean coral, Acropora palmata (commonly known as elkhorn coral) and Acropora cervicornis (staghorn coral), which until recently were the dominant reef builders in the region. The disease has so ravaged the two species that both are now listed as “critically endangered” by the International Union for Conservation of Nature. Meanwhile coral cover in the Caribbean has in recent decades declined by close to eighty percent.”

“Finally and perhaps most significant on the list of hazards is climate change—ocean acidification’s equally evil twin.

Tropical reefs need warmth, but when water temperatures rise too high, trouble ensues. The reasons for this have to do with the fact that reef-building corals lead double lives. Each individual polyp is an animal and, at the same time, a host for microscopic plants known as zooxanthellae. The zooxanthellae produce carbohydrates, via photosynthesis, and the polyps harvest these carbohydrates, much as farmers harvest corn. Once water temperatures rise past a certain point—that temperature varies by location and also by species—the symbiotic relation between the corals and their tenants breaks down. The zooxanthellae begin to produce dangerous concentrations of oxygen radicals, and the polyps respond, desperately and often self-defeatingly, by expelling them. “Without the zooxanthellae, which are the source of their fantastic colors, the corals appear to turn white—this is the phenomenon that’s become known as “coral bleaching.” Bleached colonies stop growing and, if the damage is severe enough, die. Bleached colonies stop growing and, if the damage is severe enough, die. There were major bleaching events in 1998, 2005, and 2010, and the frequency and intensity of such events are expected to increase as global temperatures climb. A study of more than eight hundred reef-building coral species, published in Science in 2008, found a third of them to be in danger of extinction, largely as a result of rising ocean temperatures. This has made stony corals one of the most endangered groups on the planet: the proportion of coral species ranked as “threatened,” the study noted, exceeds “that of most terrestrial animal groups apart from amphibians.

“ISLANDS are worlds in miniature or, as the writer David Quammen observed, “almost a caricature of nature’s full complexity.” By this account, One Tree is a caricature of a caricature. The whole place is less than 750 feet long and 500 feet wide, yet hundreds of scientists have worked there, drawn to it, in many cases, by its very diminutiveness. In the nineteen-seventies, a trio of Australian scientists set about producing a complete biological census of the island. They spent the better part of three years living in tents and cataloging every single plant and animal species they could find, including: trees (3 species), grasses (4 species), birds (29 species), flies (90 species) and mites (102 species). The island, they discovered, has no resident mammals, unless you count the scientists themselves or a pig that was once brought over and kept in a cage until it was barbecued. The monograph that resulted from this research ran to four hundred pages. It opened with a poem attesting to the charms of the tiny cay:”

“An island slumbering—

Clasped in a shimmering circlet

Of waters turquoise and blue.

Guarding her jewel from the pounding surf

On her coral rim.”

“On my last day at One Tree, no snorkeling trips were planned, so I decided to try to walk across the island, an exercise that should have taken about fifteen minutes. Not very far into my journey, I ran into Graham, the station manager. A rangy man with bright blue eyes, ginger-colored hair, and a walrus mustache, Graham looked to me like he would have made an excellent pirate. We fell into walking and talking together, and as we wandered along, Graham kept picking up bits of plastic that the waves had carried to One Tree: the cap of a bottle; a scrap of insulation, probably from a ship’s door; a stretch of PVC pipe. He had a whole collection of these bits of flotsam, which he displayed in a wire cage; the point of the exhibit, he told me, was to demonstrate to visitors “what our race is doing.”

“Graham offered to show me how the research station actually functioned, and so we threaded our way behind the cabins and the labs, toward the island’s midsection. It was breeding season, and everywhere we walked, there were birds strutting around, screaming: bridled terns, which are black on top and white on their chests; lesser crested terns, which are gray with black and white faces; and black noddies, which have a patch of white on their heads. I could see why humans had had such an easy time killing off nesting seabirds; the terns seemed completely unafraid and were so much underfoot it took an effort not to step on them.”

“Graham brought me to see the photovoltaic panels that provide the research station with power, and the tanks for collecting rain to supply it with water. The tanks were mounted on a platform, and from it we could look over the tops of the island’s trees. According to my very rough calculations, these numbered around five hundred. They seemed to be growing directly out of the rubble, like flagpoles. Just beyond the edge of the platform, Graham pointed out a bridled tern that was pecking at a black noddy chick. Soon, the chick was dead. “She won’t eat him,” he predicted, and he was right. The bridled tern walked away from the chick, who shortly thereafter was consumed by a gull. “Graham was philosophical about the episode, versions of which he had obviously seen many times; it would keep the island’s bird population from outstripping its resources.”

“That night was the first night of Hanukkah. For the holiday, someone had crafted a menorah out of a tree branch and strapped two candles onto it with duct tape. Lighted out on the beach, the makeshift menorah sent shadows skittering across the rubble. Dinner that evening was kangaroo meat, which I found surprisingly tasty, but which, the Israelis noted, was distinctly not kosher.”

“Later, I set out for DK-13 with a postdoc named Kenny Schneider. By this point, the tides had crept forward by more than two hours, so Schneider and I were scheduled to arrive at the site a few minutes before midnight. Schneider had made the journey before but still hadn’t quite mastered the workings of the GPS unit. About halfway there, we found that we had wandered off the prescribed route. The water was soon up to our chests. This made walking that much slower and more difficult, and the tide was now coming in. A variety of anxious thoughts ran through my mind. Would we be able to swim back to the station? Would we even be able to figure out the right direction to swim in? Would we finally settle the Fiji question?”

“Long after we were supposed to, Schneider and I spotted the yellow buoy of DK-13. We filled the sampling bottles and headed back. I was struck again by the extraordinary stars and the lightless horizon. I also felt, as I had several times at One Tree, the incongruity of my position. The reason I’d come to the Great Barrier Reef was to write about the scale of human influence. And yet Schneider and I seemed very, very small in the unbroken dark.”

“LIKE the Jews, the corals of the Great Barrier Reef observe a lunar calendar. Once a year, after a full moon at the start of the austral summer, they engage in what’s known as mass spawning—a kind of synchronized group sex. I was told that the mass spawning was a spectacle not to be missed, and so I planned my trip to Australia accordingly.”

“For the most part, corals are extremely chaste; they reproduce asexually, by “budding.” The annual spawning is thus a rare opportunity to, genetically speaking, mix things up. Most spawners are hermaphrodites, meaning that a single polyp produces both eggs and sperm, all wrapped together in a convenient little bundle. No one knows exactly how corals synchronize their spawning, but they are believed to respond to both light and temperature.”

“In the buildup to the big night—the mass spawning always occurs after sundown—the corals begin to “set,” which might be thought of as the scleractinian version of going into labor. The egg-sperm bundles start to bulge out from the polyps, and “the whole colony develops what looks like goose bumps. Back on Heron Island, some Australian researchers had set up an elaborate nursery so they could study the event. They had gathered up colonies of some of the most common species on the reef, including Acropora millepora, which, as one of the scientists put it to me, functions as the “lab rat” of the coral world, and were raising them in tanks. Acropora millepora produces a colony that looks like a cluster of tiny Christmas trees. No one was allowed to go near the tanks with a flashlight, for fear that it would upset the corals’ internal clocks. Instead everyone was wearing special red headlamps. With a borrowed headlamp, I could see the egg-sperm bundles straining against the polyps’ transparent tissue. The bundles were pink and resembled glass beads.”

“The head of the team, a researcher named Selina Ward, from the University of Queensland, bustled around the tanks of gravid corals like an obstetrician preparing for a delivery. She told me that each bundle held somewhere between twenty and forty eggs and probably thousands of sperm. Not long after they were released, the bundles would break open and spill their gametes, which, if they managed to find partners, would result in tiny pink larvae. As soon as the corals in her tanks spawned, Ward was planning to scoop up the bundles and subject them to different levels of acidification. She had been studying the effects of acidification on spawning for the past several years, and her results suggested that lower saturation levels led to significant “declines in fertilization. Saturation levels also affected larval development and settlement—the process by which coral larvae drop out of the water column, attach themselves to something solid, and start producing new colonies.”

“Broadly speaking, all our results have been negative so far,” Ward told me. “If we continue the way we are, without making dramatic changes to our carbon emissions immediately, I think we’re looking at a situation where, in the future, what we’ve got at best is remnant patches of corals.”

“Later that night, some of the other researchers at Heron Island, including the graduate students who were trying to weld together the overdue mesocosm, heard that Ward’s corals were getting ready to spawn and organized a nocturnal snorkel. This was a much more elaborate affair than the snorkeling trips at One Tree, complete with wet suits and underwater lights. There wasn’t enough equipment for everyone to go at once, so we went in two shifts. I was in the first, and initially I was disappointed, because nothing seemed to be happening. Then, after a while, I noticed a few corals releasing their bundles. Almost immediately, countless others followed. “The scene resembled a blizzard in the Alps, only in reverse. The water filled with streams of pink beads floating toward the surface, like snow falling upward. Iridescent worms appeared to eat the bundles, producing an eerie glow, and a slick of mauve began to form on the surface. When my shift was over, I reluctantly climbed out of the water and handed over my light.

“CHAPTER VIII

THE FOREST AND THE TREES

Alzatea verticillata”

“Trees are stunning,” Miles Silman was saying. “They are very beautiful. It’s true they take a little more appreciation. You walk into a forest, and the first thing you notice is, ‘That’s a big tree,’ or ‘That’s a tall tree,’ but when you start to think about their life history, about everything that goes into getting a tree to that spot, it’s really neat. It’s kind of like wine; once you start to understand it, it becomes more intriguing.” We were standing in eastern Peru, at the edge of the Andes, on top of a twelve-thousand-foot-high mountain, where, in fact, there were no trees—just scrub and, somewhat incongruously, a dozen or so cows, eyeing us suspiciously. The sun was sinking, and with it the temperature, but the view, in the orange glow of evening, was extraordinary. To the east was the ribbon of the Alto Madre de Dios River, which flows into the Beni River, which flows into the Madeira River, which eventually meets the Amazon. Spread out before us was Manú National Park, one of the world’s great biodiversity “hot spots.”

“In your field of vision is one out of every nine bird species on the planet,” Silman told me. “Just in our plots alone, we have over a thousand species of trees.”

Silman and I and several of Silman’s Peruvian graduate students had just arrived on the mountaintop, having set out that morning from the city of Cuzco. As the crow flies, the distance we’d traveled was only about fifty miles, but the trip had taken us an entire day of driving along serpentine dirt roads. The roads wound past villages made of mud brick and fields perched at improbable angles and women in colorful skirts and brown felt hats carrying babies in slings on their backs. At the largest of the towns, we’d stopped to have lunch and purchase provisions for a four-day hike. These included bread and cheese and a shopping bag’s worth of coca leaves that Silman had bought for the equivalent of about two dollars.”

“Standing on the mountaintop, Silman told me that the trail we were going to take down the following morning was often used by coca peddlers walking up. The cocaleros carried the “eaves from the valleys where they are grown to high Andean villages of the sort we’d just passed, and the trail had been used for this purpose since the days of the conquistadors.

Silman, who teaches at Wake Forest University, calls himself a forest ecologist, though he also answers to the title tropical ecologist, community ecologist, or conservation biologist. He began his career thinking about how forest communities are put together, and whether they tend to remain stable over time. This led him to look at the ways the climate in the tropics had changed in the past, which led him, naturally enough, to look into how it is projected to change in the future. What he learned inspired him to establish the series of tree plots that we are about to visit. Each of Silman’s plots—there are seventeen in all—sits at a different elevation and hence has a different average annual temperature. In the mega-diverse world of Manú, this means that each plot represents a slice of a fundamentally different forest community.

“In the popular imagination, global warming is mostly seen as a threat to cold-loving species, and there are good reasons for this. As the world warms, the poles will be transformed. In the Arctic, perennial sea ice covers just half the area it did thirty years ago, and thirty years from now, it may well be gone entirely. Obviously, any animal that depends on the ice—ringed seals, say, or polar bears—is going to be hard-pressed as it melts away.”

“But global warming is going to have just as great an impact—indeed, according to Silman, an even greater impact—in the tropics. The reasons for this are somewhat more complicated, but they start with the fact that the tropics are where most species actually live.”

“CONSIDER for a moment the following (purely hypothetical) journey. You are standing on the North Pole one fine spring day. (There is, for the moment, still plenty of ice at the pole, so there’s no danger of falling through.) You start to walk, or better yet ski. Because there is only one direction to move in, you have to go south, but you have 360 meridians to choose from. Perhaps, like me, you live in the Berkshires and are headed to the Andes, so you decide that you will follow the seventy-third meridian west. You ski and ski, and finally, about five hundred miles from the pole, you reach Ellesmere Island. All this time, of course, you will not have seen a tree or a land plant of any kind, since you are traveling across the Arctic Ocean. On Ellesmere, you will still not see any trees, at least not any that are recognizable as such. The only woody plant that grows on the island is the Arctic willow, which reaches no higher than your ankle. (The writer Barry Lopez has noted that if you spend much time wandering around the Arctic, you eventually realize “that you are “standing on top of a forest.

“As you continue south, you cross the Nares Strait—getting around is now becoming more complicated, but we’ll leave that aside—then traverse the westernmost tip of Greenland, cross Baffin Bay, and reach Baffin Island. On Baffin, there is also nothing that would really qualify as a tree, though several species of willow can be found, growing in knots close to the ground. Finally—and you are now roughly two thousand miles into your journey—you reach the Ungava Peninsula, in northern Quebec. Still you are north of the treeline, but if you keep walking for another 250 miles or so, you will reach the edge of the boreal forest. Canada’s boreal forest is huge; it stretches across almost a billion acres and represents roughly a quarter of all the intact forest that remains on earth. But diversity in the boreal forest is low. Across Canada’s billion acres of it, you will find only about twenty species of tree, including black spruce, white birch, and balsam fir.”

“Once you enter the United States, tree diversity will begin, slowly, to tick up. In Vermont, you’ll hit the Eastern Deciduous Forest, which once covered almost half the country, but today remains only in patches, most of them second-growth. Vermont has something like fifty species of native trees, Massachusetts around fifty-five. North Carolina (which lies slightly to the west of your path) has more than two hundred species. Although the seventy-third meridian misses Central America altogether, it’s “worth noting that tiny Belize, which is about the size of New Jersey, has some seven hundred native tree species.”

“The seventy-third meridian crosses the equator in Colombia, then slices through bits of Venezuela, Peru, and Brazil before entering Peru again. At around thirteen degrees south latitude, it passes to the west of Silman’s tree plots. In his plots, which collectively have an area roughly the size of Manhattan’s Fort Tryon Park, the diversity is staggering. One thousand and thirty-five tree species have been counted there, roughly fifty times as many as in all of Canada’s boreal forest.”

“And what holds for the trees also holds for birds and butterflies and frogs and fungi and just about any other group you can think of (though not, interestingly enough, for aphids). As a general rule, the variety of life is most impoverished at the poles and richest at low latitudes. This pattern is referred to in the scientific literature as the “latitudinal diversity gradient,” or LDG, and it was noted already by the German naturalist Alexander von Humboldt, who was amazed by the biological splendors of the tropics, which offer “a spectacle as varied as the azure vault of the heavens.”

“The verdant carpet which a luxuriant Flora spreads over the surface of the earth is not woven equally in all parts,” Humboldt wrote after returning from South America in 1804. “Organic development and abundance of vitality gradually increase from the poles towards the equator.” More than two centuries later, why this should be the case is still not known, though more than thirty theories have been advanced to explain the phenomenon.”

“One theory holds that more species live in the tropics because the evolutionary clock there ticks faster. Just as farmers can produce more harvests per year at lower latitudes, organisms can produce more generations. The greater the number of generations, the higher the chances of genetic mutations. The higher the chances of mutations, the greater the likelihood that new species will emerge. (A slightly different but related theory has it that higher temperatures in and of themselves lead to higher mutation rates.)”

“A second theory posits that the tropics hold more species because tropical species are finicky. According to this line of reasoning, what’s important about the tropics is that temperatures there are relatively stable. Thus tropical organisms tend to possess relatively narrow thermal tolerances, and even slight climatic differences, caused, say, by hills or valleys, can constitute insuperable barriers. (A famous paper on this subject is titled “Why Mountain Passes Are Higher in the Tropics.”) Populations are thus more easily isolated, and speciation ensues.”

“Yet another theory centers on history. According to this account, the most salient fact about the tropics is that they are old. A version of the Amazon rainforest has existed for many millions of years, since before there even was an Amazon. Thus, “in the tropics, there’s been lots of time for diversity to, as it were, accumulate. By contrast, as recently as twenty thousand years ago, nearly all of Canada was covered by ice a mile thick. So was much of New England, meaning that every species of tree now found in Nova Scotia or Ontario or Vermont or New Hampshire is a migrant that’s arrived (or returned) just in the last several thousand years. The diversity as a function of time theory was first advanced by Darwin’s rival, or, if you prefer, codiscoverer, Alfred Russel Wallace, who observed that in the tropics “evolution has had a fair chance,” while in glaciated regions “it has had countless difficulties thrown in its way.

“THE following morning, we all crawled out of our sleeping bags early to see the sunrise. Overnight, clouds had rolled in from the Amazon basin, and we watched them from above as they turned first pink and then flaming orange. In the chilly dawn, we packed up our gear and headed down the trail. “Pick out a leaf with an interesting shape,” Silman instructed me once we’d descended into the cloud forest. “You’ll see it for a few hundred meters, and then it will be gone. That’s it. That’s the tree’s entire range.”

“Silman was carrying a two-foot-long machete, which he used to hack away at the undergrowth. Occasionally, he waved it in the air to point out something interesting: a spray of tiny white orchids with flowers no bigger than a grain of rice; a plant in the blueberry family with vivid red berries; a parasitic shrub with bright orange flowers. One of Silman’s graduate students, William Farfan Rios, handed me a leaf the size of a dinner plate.”

“This is a new species,” he said. Along the trail, Silman and his students have found thirty species of trees new to science. (Just this grove of discoveries represents half again as many species as in Canada’s boreal forest.) And there are another three hundred species that they suspect may be new, but that have yet to be formally classified. What’s more, they’ve discovered an entirely new genus.”

“That’s not like finding another kind of oak or another kind of hickory,” Silman observed. “It’s like finding ‘oak’ or ‘hickory.’” Leaves from trees in the genus had been sent to a specialist at the University of California-Davis, but, unfortunately, he had died before figuring out where on the taxonomic tree to stick the new branch.

Although it was winter in the Andes and the height of the dry season, the trail was muddy and slick. It had worn a deep channel into the mountainside, so that as we walked along, the ground was at eye level. At various points, trees had grown across the top and the channel became a tunnel. The first tunnel we hit was dark and dank and dripping with fine rootlets. Later tunnels were longer and darker and even in the middle of the “day required a headlamp to navigate. Often I felt as if I’d entered into a very grim fairy tale.

We passed Plot 1, elevation 11,320 feet, but did not stop there. Plot 2, elevation 10,500 feet, had been recently scoured by a landslide; this pleased Silman because he was interested to see what sorts of trees would recolonize it.

“The farther we descended, the denser the forest became. The trees were not just trees; they were more like botanical gardens, covered with ferns and orchids and bromeliads and strung with lianas. In some spots, the vegetation was so thick that soil mats had formed above the ground, and these had sprouted plants of their own—forests in the air. With nearly every available patch of light and bit of space occupied, the competition for resources was evidently fierce, and it almost seemed possible to watch natural selection in action, “daily and hourly” scrutinizing “every variation, even the slightest.” (Another theory of why the tropics are so diverse is that greater competition has pushed species to become more specialized, and more specialists can coexist in the same amount of space.) I could hear birds calling, but only rarely could I spot them; it was difficult to see the animals for the trees.”

“Somewhere around Plot 3, elevation 9,680 feet, Silman pulled out the shopping bag full of coca leaves. He and his students were carrying what seemed to me to be a ridiculous amount of heavy stuff: a bag of apples, a bag of oranges, a seven-hundred-page bird book, a nine-hundred-page plant book, an iPad, bottles of benzene, a can of spray paint, a wheel of cheese, a bottle of rum. Coca, Silman told me, made a heavy pack feel lighter. It also staved off hunger, alleviated aches and pains, and helped counter altitude sickness. I had been given little to carry besides “my own gear; still, anything that would lighten my pack seemed worth trying. I took a handful of leaves and a pinch of baking soda. (Baking soda, or some other alkaline substance, is necessary for coca to have its pharmaceutical effect.) The leaves were leathery and tasted like old books. Soon my lips grew numb, and my aches and pains began to fade. An hour or two later, I was back for more. (Many times since have I wished for that shopping bag.)

“In the early afternoon, we reached a small, soggy clearing where, I was informed, we were going to spend the night. This was the edge of Plot 4, elevation 8,860 feet. Silman and his students had often camped there before, sometimes for weeks at a stretch. The clearing was strewn with bromeliads that had been pulled down and gnawed upon. Silman identified these as the leavings of a spectacled bear. The spectacled bear, also known as the Andean bear, is South America’s last surviving bear. It is black or dark brown with beige around its eyes, and it lives mainly off plants. I hadn’t realized that there were bears in the Andes, and I couldn’t help thinking of Paddington, arriving in London from “deepest, darkest Peru.”

“EACH of Silman’s seventeen tree plots is two and a half acres, and the plots are arranged along a ridge a bit like buttons on a cloak. They run from the top of the ridge all the way down to the Amazon basin, which is pretty much at sea level. In the plots, someone—Silman or one of his graduate students—has tagged every single tree over four inches in diameter. Those trees have been measured, identified by species, and given a number. Plot 4 has 777 trees over four inches, and these belong to sixty different species. Silman and his students were preparing to recensus the plots, a project that was expected to take several months. All the trees that had already been tagged would have to be remeasured, and any tree that had shown up or died since the last count would have to be added or subtracted. “There were long, Talmudic discussions, conducted partly in English and partly in Spanish, about how, exactly, the recensus should be conducted. One of the few that I could follow centered on asymmetry. A tree trunk is not perfectly circular, so depending on how you orient the calipers when you’re measuring, you’ll get a different diameter. Eventually, it was decided that the calipers should be oriented with their fixed jaw on a dot spray-painted on every tree in red.

“Owing to the differences in elevation, each of Silman’s plots has a different average annual temperature. For example, in Plot 4 the average is fifty-three degrees. In Plot 3, which is about eight hundred feet higher, it’s fifty-one degrees, and in Plot 5, which is about eight hundred feet lower, it’s fifty-six degrees. Because tropical species tend to have narrow thermal ranges, these temperature differences translate into a high rate of turnover; trees that are abundant in one plot may be missing entirely from the next one down or up.”

“Some of the dominants have the narrowest altitudinal range,” Silman told me. “This suggests that what makes them such good competitors in this range makes them not so good outside of it.” In Plot 4, for example, ninety percent of the tree species are different from those species found in Plot 1, which is only about twenty-five hundred feet higher.”

“Silman first laid out the plots in 2003. His idea was to keep coming back, year after year, decade after decade, to see what happened. How would the trees respond to climate change? One possibility—what might be called the Birnam Wood scenario—was that the trees in each zone would start moving upslope. Of course, trees can’t actually move, but they can do the next best thing, which is to disperse seeds that grow into new trees. Under this scenario, species now found in Plot 4 would, as the climate warmed, start appearing higher upslope, in Plot 3, while Plot 3’s would appear in Plot 2, and so on. Silman and his students completed the first recensus in 2007. Silman thought of the effort as part of his long-term project and couldn’t imagine that much of interest would be found after just four years. But one of his postdocs, Kenneth Feeley, insisted on sifting through all the data, anyway. Feeley’s work revealed that the forest was already, measurably, in motion.”

“There are various ways to calculate migration rates: for instance, by the number of trees or, alternatively, by their mass. Feeley grouped the trees by genus. Very roughly speaking, he found that global warming was driving the average genus up the mountain at a rate of eight feet per year. But he also found the average masked a surprising range of response. Like cliques of kids at recess, different trees were behaving in wildly different ways.

“Take, for example, trees in the genus Schefflera. Schefflera, which is part of the ginseng family, has palmately compound leaves; these are arrayed around a central point the way your fingers are arranged around your palm. (One member of the group, Schefflera arboricola, from Taiwan, commonly known as the dwarf umbrella tree, is often grown as a houseplant.) Trees in Schefflera, Feeley found, were practically hyperactive; they were racing up the ridge at the astonishing rate of nearly a hundred feet a year.

On the opposite extreme were trees in the genus Ilex. These have alternate leaves that are usually glossy, with spiky or serrated edges. (The genus includes Ilex aquifolium, which is native to Europe and known to Americans as Christmas holly.) The trees in Ilex were like kids who spend recess sprawled out on a bench. While Schefflera was sprinting upslope, Ilex was just sitting there, more or less inert.”

“ANY species (or group of species) that can’t cope with some variation in temperatures is not a species (or group) whose fate we need be concerned about right now, because it no longer exists. Everywhere on the surface of the earth temperatures fluctuate. They fluctuate from day to night and from season to season. Even in the tropics, where the difference between winter and summer is minimal, temperatures can vary significantly between the rainy and the dry seasons. Organisms have developed all sorts of ways of dealing with these variations. They hibernate or estivate or migrate. They dissipate heat through panting or conserve it by growing thicker coats of fur. Honeybees warm themselves by contracting the muscles in their thorax. Wood storks cool off by defecating on their own legs. (In very hot weather, wood storks may excrete on their legs as often as once a minute.)

“Over the lifetime of a species, on the order of a million years, longer-term temperature changes—changes in climate—come into play. For the last forty million years or so, the earth has been in a general cooling phase. It’s not entirely clear why this is so, but one theory has it that the uplift of the Himalayas exposed vast expanses of rock to chemical weathering, and this in turn led to a drawdown of carbon dioxide from the atmosphere. At the start of this long cooling phase, in the late Eocene, the world was so warm there was almost no ice on the planet. By around thirty-five million years ago, global temperatures had declined enough that glaciers began to form on Antarctica. By three million years ago, temperatures had dropped to the point that the Arctic, too, froze over, and a permanent ice cap formed. Then, about two and a half million years “ago, at the start of the Pleistocene epoch, the world entered a period of recurring glaciations. Huge ice sheets advanced across the Northern Hemisphere, only to melt away again some hundred thousand years later.”

“Even after the idea of ice ages was generally accepted—it was first proposed in the eighteen-thirties by Louis Agassiz, a protégé of Cuvier—no one could explain how such an astonishing process could take place. In 1898, Wallace observed that “some of the most acute and powerful intellects of our day have exerted their ingenuity” on the problem, but so far “altogether in vain.” It would take another three-quarters of a century for the question to be resolved. It is now generally believed that ice ages are initiated by small changes in the earth’s orbit, caused by, among other things, the gravitational tug of Jupiter and Saturn. These changes alter the distribution of sunlight across different latitudes at different times of year. When the amount of light hitting the far northern latitudes in summer approaches a minimum, snow begins to build up there. “This initiates a feedback cycle that causes atmospheric carbon dioxide levels to drop. Temperatures fall, which leads more ice to build up, and so on. After a while, the orbital cycle enters a new phase, and the feedback loop begins to run in reverse. The ice starts to melt, global CO2 levels rise, and the ice melts back farther.

During the Pleistocene, this freeze-thaw pattern was repeated some twenty times, with world-altering effects. “So great was the amount of water tied up in ice during each glacial episode that sea levels dropped by some three hundred feet, and the sheer weight of the sheets was enough to depress the crust of the earth, pushing it down into the mantle. (In places like northern Britain and Sweden, the process of rebound from the last glaciation is still going on.)”

“How did the plants and animals of the Pleistocene cope with these temperature swings? According to Darwin, they did so by moving. In On the Origin of Species, he describes vast, continental-scale migrations.

As the cold came on, and as each more southern zone became fitted for arctic beings and ill-fitted for their former more temperate inhabitants, the latter would be supplanted and arctic productions would take their places.… As the warmth returned, the arctic forms would retreat northward, closely followed up in their retreat by the productions of the more temperate regions.

“Darwin’s account has since been confirmed by all sorts of physical traces. Researchers studying ancient beetle casings, for example, have found that during the ice ages, even tiny insects migrated thousands of miles to track the climate. (To name just one of these, Tachinus caelatus is a small, dullish brown beetle “that today lives in the mountains west of Ulan Bator, in Mongolia. During the last glacial period, it was common in England.)”

“In its magnitude, the temperature change projected for the coming century is roughly the same as the temperature swings of the ice ages. (If current emissions trends continue, the Andes are expected to warm by as much as nine degrees.) But if the magnitude of the change is similar, the rate is not, and, once again, rate is key. Warming today is taking place at least ten times faster than it did at the end of the last glaciation, and at the end of all those glaciations that preceded it. To keep up, organisms will have to migrate, or otherwise adapt, at least ten times more quickly. In Silman’s plots, only the most fleet-footed (or rooted) trees, like the hyperactive genus Schefflera, are keeping pace with rising temperatures. How many species overall will be capable of moving fast enough remains an open question, though, as Silman pointed out to me, in the coming decades we are probably going to learn the answer, whether we want to or not.”

“MANÚ National Park, where Silman’s plots are laid out, sits in the southeastern corner of Peru, near the country’s borders with Bolivia and Brazil, and it stretches over nearly six thousand square miles. According to the United Nations Environment Programme, Manú is “possibly the most biologically diverse protected area in the world.” Many species can be found only in the park and its immediate environs; these include the tree fern Cyathea multisegmenta, a bird known as the white-cheeked tody flycatcher, a rodent called Barbara Brown’s brush-tailed rat, and a small, black toad known only by its Latin name, Rhinella manu.”

“The first night on the trail, one of Silman’s students, Rudi Cruz, insisted that we all go out looking for Rhinella manu. He had seen several of the toads during a previous visit to the spot, and he felt sure we could find them again if we tried. I’d recently read a paper on the spread of the chytrid fungus to Peru—according to the authors, it had already arrived in Manú—but I decided not to mention this. Perhaps Rhinella manu was still out there, in which case I certainly wanted to see it.”

“We strapped on headlamps and set out down the trail, like a line of coal miners filing down a shaft. The forest at night had become an impenetrable tangle of black. Cruz led the way, shining his lamp along the tree trunks and peering into the bromeliads. The rest of us followed suit. This went on for maybe an hour and turned up only a few brownish frogs from the genus Pristimantis. After a while, people started getting bored and drifting back to camp. Cruz refused to give up. Perhaps thinking that the problem was the rest of us, he headed up the trail in the opposite direction. “Did you find anything?” someone would periodically call out to him through the darkness.

“Nada,” came the repeated response.”

“The next day, after more arcane discussions about tree measurements, we packed up to continue down the ridge. On a trip to fetch water, Silman had found a spray of white berries interspersed with what looked like bright purple streamers. He’d identified the arrangement as the inflorescence of a tree in the Brassicaceae, or mustard, family, but he had never seen anything quite like it before, which made him think, he told me, that it might represent yet another new species. It was pressed in newspaper for transport down the mountain. The idea that I might have been present at the discovery of a species, even though I’d had absolutely nothing to do with it, filled me with an odd sort of pride.”

“BACK on the trail, Silman hacked away with his machete, pausing every now and then to point out a new botanical oddity, like a shrub that steals water from its neighbors by sticking out needlelike roots. Silman talks about plants the way other people speak about movie stars. One tree he described to me as “charismatic.” Others were “hilarious,” “crazy,” “neat,” “clever,” and “amazing.”

“Sometime in the mid-afternoon, we emerged onto a rise with a view across a valley to the next ridge. On the ridge, the trees were shaking. This was a sign of woolly monkeys making their way through the forest. Everyone stopped to try to get a glimpse of them. As they sailed from branch to branch, the monkeys made a chirruping noise, a bit like the whine of crickets. Silman pulled out the shopping bag and passed it around.”

“A little while later, we reached Plot 6, elevation 7,308 feet, where the tree from the new genus had been found. Silman waved his machete at it. The tree looked pretty ordinary, but I tried to see it through his eyes. It was taller than most of its neighbors—perhaps it could be described as “stately” or “statuesque”—with smooth, ruddy bark and simple, alternate leaves. It belonged to the Euphorbiaceae, or spurge, family, whose members include poinsettia. Silman was eager to learn as much as possible about the tree, so that when a new taxonomist could be found to replace the one who had died, he’d be able to send him all the necessary material. He and Farfan went to see what they could come up with. They returned with some seed capsules, which were as thick and tough as hazelnut shells, but delicately shaped, like flowering lilies. The capsules were dark brown on the outside and inside the color of sand.”

“That evening, the sun set before we reached Plot 8, where we were going to camp. We hiked on through the dark, then set up our tents and made dinner, also in the dark. I crawled into my sleeping bag around 9 PM, but a few hours later, I was woken by a light. I assumed someone had gotten up to pee, and rolled over. In the morning, Silman told me that he was surprised I’d been able to sleep through all the commotion. “Six groups of cocaleros had tromped through the campsite overnight. (In Peru, though the sale of coca is legal, all purchases are supposed to go through a government agency known as ENACO, a restriction growers do their best to avoid.) Every single group had tripped over his tent. Eventually he’d gotten so annoyed, he’d yelled at the cocaleros, which, he had to admit, probably hadn’t been the wisest idea.”

“IN ecology, rules are hard to come by. One of the few that’s universally accepted is the “species-area relationship,” or SAR, which has been called the closest thing the discipline has to a periodic table. In its broadest formulation, the species-area relationship seems so simple as to be almost self-evident. The larger the area you sample, the greater the number of species you will encounter. This pattern was noted all the way back in the seventeen-seventies by Johann Reinhold Forster, a naturalist who sailed with Captain Cook on his second voyage, the one after his unfortunate collision with the Great Barrier Reef. In the nineteen-twenties, it was codified mathematically by a Swedish botanist, Olof Arrhenius. (As it happens, Olof was the son of the chemist Svante Arrhenius, who, in the eighteen-nineties, showed that burning fossil fuels would lead to a warmer planet.) And it was further refined and elaborated by E. O. Wilson and his colleague Robert MacArthur in the nineteen-sixties.”

“The correlation between the number of species and the size of the area is not linear. Rather, it’s a curve that slopes in a predictable way. Usually, the relationship is expressed by the formula S = cAz, where S is the number of species, A is the size of the area, and c and z are constants that vary according to the region and taxonomic group under consideration (and hence are not really constants in the usual sense of the term). The relationship counts as a rule because the ratio holds no matter what the terrain. You could be studying a chain of islands or a rainforest or a nearby state park, and you’d find that the number of species varies according to the same insistent equation: S = cAz.*”

“For the purposes of thinking about extinction, the species-area relationship is key. One (admittedly simplified) way of conceiving of what humans are doing to the world is that we are everywhere changing the value of A. Consider, for example, a grassland that once covered a thousand square miles. Let’s say the grassland was home to a hundred species of birds (or beetles or snakes). If half of the grassland were eliminated—converted into farmland or shopping malls—it should be possible to calculate, using the species-area relationship, the proportion of bird species (or beetles or snakes) that would be lost. Very roughly speaking, the answer is ten percent. (Here again, it’s important to remember that the relationship is not linear.) Since it takes a long time for the system to reach a new equilibrium, you wouldn’t expect the species to disappear right away, but you would expect them to be headed in that direction.”

“In 2004, a group of scientists decided to use the species-area relationship to generate a “first-pass” estimate of the extinction risk posed by global warming. First, the members of the team gathered data on the current ranges of more than a thousand plant and animal species. Then they correlated these ranges with present-day climate conditions. Finally, they imagined two extreme scenarios. In one, all of the species were assumed to be inert, much like the Ilex trees in Silman’s plots. “As temperatures rose, they stayed put, and so, in most cases, the amount of climatically suitable area available to them shrank, in many instances down to zero. The projections based on this “no dispersal” scenario were bleak. If warming were held to a minimum, the team estimated that between 22 and 31 percent of the species would be “committed to extinction” by 2050. If warming were to reach what was at that point considered a likely maximum—a figure that now looks too low—by the middle of this century, between 38 and 52 percent of the species would be fated to disappear.”

“Here’s another way to express the same thing,” Anthony Barnosky, a paleontologist at the University of California-Berkeley, wrote of the study results. “Look around you. Kill half of what you see. Or if you’re feeling generous, just kill about a quarter of what you see. That’s what we could be talking about.”

“In the second, more optimistic scenario, species were imagined to be highly mobile. Under this scenario, as temperatures climbed, creatures were able to colonize any new areas that met the climate conditions they were adapted to. Still, many species ended up with nowhere to go. As the earth warmed, the conditions they were accustomed to simply disappeared. (The “disappearing climates” turned out to be largely in the tropics.) Other species saw their habitat shrink because to track the climate they had to move upslope, and the area at the top of a mountain is smaller than at the base.”

“Using the “universal dispersal” scenario, the team, led by Chris Thomas, a biologist at the University of York, found that, with the minimum warming projected, 9 to 13 percent of all species would be “committed to extinction” by 2050. With maximum warming, the numbers would be 21 to 32 percent. Taking the average of the two scenarios, and looking at a mid-range warming projection, the group concluded that 24 percent of all species would be headed toward extinction.”

“The study ran as the cover article in Nature. In the popular press, the welter of numbers the researchers came up with was condensed down to just one. “Climate Change Could Drive a Million of the World’s Species to Extinction,” the BBC declared. “By 2050 Warming to Doom a Million Species” is how the headline in National Geographic put it.”

“The study has since been challenged on a number of grounds. It ignores interactions between organisms. It doesn’t account for the possibility that plants and animals can tolerate a broader range of climates than their current range suggests. It looks only as far as 2050 when, under any remotely plausible scenario, warming will continue far beyond that. It applies the species-area relationship to a new, and therefore untested, set of conditions.”

“More recent studies have come down on both sides of the Nature paper. Some have concluded that the paper overestimated the number of extinctions likely to be caused by climate change, others that it understated it. For his part, Thomas has acknowledged that many of the objections to the 2004 paper may be valid. But he has pointed out that every estimate that’s been proposed since then has been the same order of magnitude. Thus, he’s observed, “around 10 or more percent of species, and not 1 percent, or .01 percent,” are likely to be done in by climate change.”

“In a recent article, Thomas suggested that it would be useful to place these numbers “in a geological context.” Climate change alone “is unlikely to generate a mass extinction as large as one of the Big Five,” he wrote. However, there’s a “high likelihood that climate change on its own could generate a level of extinction on par with, or exceeding, the slightly ‘lesser’ extinction events” of the past.

“The potential impacts,” he concluded, “support the notion that we have recently entered the Anthropocene.”

Elizabeth Kolbert. “The Sixth Extinction.