The Sixth Extinction: An Unnatural History

by Elizabeth Kolbert

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.

In the early years of the nineteenth century, fossil collecting became so popular among the upper classes that a whole new vocation sprang up.

CUVIER’S ideas about this history of life—that it was long, mutable, and full of fantastic creatures that no longer existed—would seem to have made him a natural advocate for evolution. But Cuvier opposed the concept of evolution, or transformisme as it was known in Paris at the time, and he tried—generally, it seems, successfully—to humiliate any colleagues who advanced the theory.

Jean-Baptiste Lamarck. According to Lamarck, there was a force—the “power of life”—that pushed organisms to become increasingly complex. At the same time, animals and also plants often had to cope with changes in their environment. They did so by adjusting their habits; these new habits, in turn, produced physical modifications that were then passed down to their offspring.

Lamarck, for his part, adamantly opposed Cuvier’s idea of extinction; there was no process he could imagine capable of wiping an organism out entirely. (Interestingly, the only exception he entertained was humanity, which, Lamarck allowed, might be able to exterminate certain large and slow-to-reproduce animals.)

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.

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.

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,

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 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.

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.”

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.

The International Commission on Stratigraphy, or ICS, is the group responsible for maintaining the official timetable of earth’s history.

Carbon dioxide has many interesting properties, one of which is that it dissolves in water to form an acid.

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.

“Time is the essential ingredient, but in the modern world there is no time.”

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.

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.)

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.

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.”

ISLANDS—WE are talking about real islands now, rather than “islands” of habitat—tend to be species-poor, or, to use the term of art, depauperate.

They keep on bleeding species—a process that’s known by the surprisingly sunny term “relaxation.”

In the absence of recolonization, local extinctions can become regional and then, eventually, global.

diversity tends to be self-reinforcing. “A natural corollary to high species diversity is low population density, and that’s a recipe for speciation—isolation by distance,” he explained. It’s also, he added, a vulnerability, since small, isolated populations are that much more susceptible to extinction.

“If we look far enough ahead, the eventual state of the biological world will become not more complex, but simpler—and poorer,” Elton wrote.

“Chimps do a lot of incredibly smart things,” Michael Tomasello, who heads the institute’s department of developmental and comparative psychology, told me. “But the main difference we’ve seen is ‘putting our heads together.’ If you were at the zoo today, you would never have seen two chimps carry something heavy together. They don’t have this kind of collaborative project.”

Archaic humans like Homo erectus “spread like many other mammals in the Old World,” Pääbo told me. “They never came to Madagascar, never to Australia. Neither did Neanderthals. It’s only fully modern humans who start this thing of venturing out on the ocean where you don’t see land. Part of that is technology, of course; you have to have ships to do it. But there is also, I like to think or say, some madness there. You know? How many people must have sailed out and vanished on the Pacific before you found Easter Island? I mean, it’s ridiculous. And why do you do that? Is it for the glory? For immortality? For curiosity? And now we go to Mars. We never stop.”

the qualities that made us human to begin with: our restlessness, our creativity, our ability to cooperate to solve problems and complete complicated tasks.

“In many ways human language is like the genetic code,” the British paleontologist Michael Benton has written. “Information is stored and transmitted, with modifications, down the generations. Communication holds societies together and allows humans to escape evolution.”

having freed ourselves from the constraints of evolution, humans nevertheless remain dependent on the earth’s biological and geochemical systems. By disrupting these systems—cutting down tropical rainforests, altering the composition of the atmosphere, acidifying the oceans—we’re putting our own survival in danger.

As of 2020, the IUCN had evaluated virtually all known bird species and more than ninety percent of known mammal species. It had evaluated fewer than one percent of described insect species—and described insect species, of which there are about a million, probably make up only a fraction of those that are out there.

E. O. Wilson famously put it: “If all mankind were to disappear, the world would regenerate back to the rich state of equilibrium that existed ten thousand years ago. If insects were to vanish, the environment would collapse into chaos.”

David Wagner, an entomologist at the University of Connecticut, had organized the session. When it was his turn at the mic, he pointed to a “conundrum.” The speakers, he noted, had all pretty much agreed insects were in trouble, but when it came to a cause, there was no consensus: some blamed climate change, others farming practices. “It’s pretty phenomenal that we have so many scientists looking at this problem and yet are not exactly certain what the stressors are,” he observed.