THE DINOSAURS WERE THE LEAST OF IT. They, together with other “charismatic megafauna,” went extinct during a massive global event at the end of the Cretaceous period, sixty-six million years ago—but by then there had already been four other mass extinctions, dating as far back as the Ordovician period, 444 million years ago. And now, according to New Yorker staff writer Elizabeth Kolbert, we’re heading for another: a sixth extinction, which she characterizes as “the amazing moment that to us counts as the present, [when] we are deciding, without quite meaning to, which evolutionary pathways will remain open and which will forever be closed.”
By extinction, Kolbert means the massive die-off of almost everything. In this dire state, “the usual rules of survival are suspended,” she writes. “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 when dealing with ordinary threats may turn out, under such extraordinary circumstances, to be fatal.”
The Ordovician extinction was probably caused by glaciation. Next was the Devonian extinction, about 370 million years ago, followed by the biggest of them all, the Permian, when intense global warming made oceans heat up as much as eighteen degrees and some 90 percent of all species disappeared, coming “scarily close to eliminating multi-cellular life altogether.” The fourth extinction occurred in the Jurassic period, and the fifth was the one that most captures the popular imagination—the end of the dinosaurs, probably caused by an asteroid that fell to Earth sometime in the late Cretaceous. Most nondinosaur species were obliterated during the Cretaceous extinction, too: Virtually every land animal “larger than a cat” died off, as well as 75 percent of all birds, 80 percent of lizards and snakes, and 95 percent of plankton. The list of organisms lost forever in the fifth extinction includes some wonderful ones: plesiosaurs, mosasaurs, belemnites, ammonites, brachiopods, bryozoans—and those are just species that lived in the sea.
The idea that mass extinction was even possible has only lately dawned on Homo sapiens, in the larger scheme of things. First, scientists had to come to a general consensus that the history of the earth is one of gradual, incremental change. This idea didn’t come naturally: “Aristotle wrote a ten-book History of Animals without ever considering the possibility that animals actually had a history,” Kolbert writes. Even as enlightened a thinker as Thomas Jefferson, when confronted with evidence that a mastodon unlike any animal then extant had once roamed America, rejected the concept outright. As he wrote in 1781: “Such is the economy of nature 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.”
It was not until the time of the French Revolution that the naturalist Georges Cuvier insisted that the earth had a history, and that some species of animals that had existed in the past no longer existed in the present. Cuvier was an unlikely hero, according to Kolbert; he “could be charming and he could be vicious; he was a visionary and, at the same time, a reactionary.” He was vilified in his lifetime for his insistence that there was a “world previous to ours” populated by vanished species now seen only in the fossils he obsessively collected. But, Kolbert writes, his “essentially tragic vision of earth history has come to seem prophetic.”
In a wonderfully vivid chapter on the evolution of the idea of evolution (a theory in which Cuvier, so prescient in other ways, refused to believe), Kolbert introduces us to some of the major players who helped to assemble a historical account of how species have changed—and died out—over time: the geologist Charles Lyell, the naturalists John Wolley and Alfred Newton, and, of course, the originator of the concept of natural selection, Charles Darwin. She lobs some mild criticism at Darwin for his failure to acknowledge and condemn the scourge of human-caused extinction. Even in his own lifetime, he had witnessed the disappearance, due to human activity, of two species, the great auk and the Charles Island tortoise. “It’s puzzling,” she writes, “that a writer as shrewd and self-critical as Darwin” should find “nothing remarkable or troubling about this.”
For aid in keeping track of the geological periods she discusses, Kolbert offers a mnemonic: Camels Often Sit Down Carefully, Perhaps Because Their Joints Creak. This bizarre sentence—just bizarre enough to be memorable—alludes to the first letters of the geological periods, in chronological order: Cambrian, Ordovician, Silurian, Devonian, Carboniferous, Permian, Triassic, Jurassic, Cretaceous. The sentence “unfortunately runs out” before it gets to the more recent periods of Paleogene, Neogene, and Quaternary—maybe the camel joints are creaking Primarily Near Quahogs?
The truth is, ending at Quahogs might not get us far enough. Quaternary is the accepted term for the period in which we live, but Kolbert makes a compelling case for an amendment. With Homo sapiens causing changes in climate and geology at superspeed, we are producing an enormous impact on many things, including the future of the world’s flora and fauna. Therefore, some scientists argue, our period deserves a special (and not necessarily complimentary) label: the Anthropocene. According to Kolbert, in 2016 the group that maintains the “official timetable of the earth’s history,” the International Commission on Stratigraphy, will decide whether Anthropocene should be accepted as the new name for the current epoch.
The factoids Kolbert tosses off about nature’s incredible variety—a frog that carries eggs in its stomach and gives birth through its mouth, a wood stork that cools off by defecating on its own legs—make it heartbreakingly clear, without any heavy-handed sermonizing from the author, just how much we lose when an animal goes extinct. In the same way, her intrepid reporting from far-off places—Panama, Iceland, Italy, Scotland, Peru, the Amazonian rain forest of Brazil, and the remote One Tree Island, off the coast of Australia—gives us a sense of the earth’s vastness and beauty. We get a sense of its danger, too, when Kolbert lets us in on her anxiety about entering caves, climbing cliffs, or diving into oceans alongside the scientists she shadows. She gamely jumps into the Tyrrhenian Sea one January day, for instance, with scientists studying the water’s CO2-spewing vents to preview what will happen to oceans as carbon levels rise across the globe. And as her limbs gradually grow numb from the cold, she makes passing reference to her own squirmy thoughts: “Here it is possible to swim—and even, I think in a moment of panic, to drown—in the seas of tomorrow today.”
Climate-change deniers might use Kolbert’s careful rendition of previous ebbs and flows in the temperature of the earth’s air and water to prove that what’s happening now is perfectly natural and has nothing to do with human activity. And yes, the history of the earth is characterized by massive changes in climate, and tremendous die-offs of entire species and even families of animals and plants. In the late Eocene, forty million years ago, for instance, the earth was so warm that there was almost no ice on the planet. Five million years later, the earth had cooled sufficiently for glaciers to begin to form in Antarctica; thirty million years after that (about three million years ago), there was a permanent ice cap at both the North and South Poles. At the start of the Pleistocene, 2.5 million years ago, “the world entered a period of recurring glaciations,” Kolbert writes. “Huge ice sheets advanced across the Northern Hemisphere, only to melt away again some hundred thousand years later.” This freeze-thaw cycle was dramatic, with sea levels dropping four hundred feet at each glaciation; the sheer weight of the ice was sufficient to lower the crust of the earth. But it all occurred so slowly—about twenty times over the course of some two million years—that plants and animals were able to adjust by massive migrations to regions where the environment was more tolerable, or via the selection of more adaptive traits over the course of a few generations.
That’s the crucial difference—the rate of change. Today, because of the intensity of human activity, environmental changes are happening so quickly that there might not be time for corrective migrations or other adaptation strategies. Over the next century, a temperature swing of roughly the same magnitude as that of the ice ages is projected to occur—but at a speed that’s at least ten times faster than anything the earth has seen before. “To keep up, organisms will have to migrate, or otherwise adapt, at least ten times more quickly,” Kolbert writes. And there’s no evidence that plants and animals will be able to do that. Kolbert does put some faith in the prospect of giant rats being up to the task, but not much else. Short of a world populated by rats the size of elephants—and, in one particularly gruesome image, by human-size hairless rats “living in caves, shaping rocks as primitive tools and wearing the skins of other mammals that they have killed and eaten”—Kolbert doesn’t offer much to look forward to. In her final assessment of where we’re headed—a chapter called “The Thing with Feathers”—she quotes two scientists whose points of view might fairly be called ironic. Anthropologist Richard Leakey, she tells us, said that “Homo sapiens might not only be the agent of the sixth extinction, but also risks being one of its victims.” And ecologist Paul Ehrlich put it even more bluntly. “In pushing other species to extinction,” he wrote, “humanity is busy sawing off the limb on which it perches.”
Robin Marantz Henig is a contributing writer for the New York Times Magazine whose most recent book, Twentysomething: Why Do Young Adults Seem Stuck? (Hudson Street, 2012), was cowritten with her daughter Samantha Henig.