The Sixth Extinction: An Unnatural History

by Elizabeth Kolbert

“Unfortunately,” he told me, “we are losing all these amphibians before we even know that they exist.”

“Even the regular people in El Valle, they notice it,” he said. “They tell me, ‘What happened to the frogs? We don’t hear them calling anymore.’”

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.

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.

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.

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.

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.

Keep digging and mammals disappeared altogether from the fossil record. Eventually one reached a world not just previous to ours, but a world previous to that, dominated by giant reptiles.

Having dismissed transformisme, Cuvier was left with a gaping hole. He had no account of how new organisms could appear, nor any explanation of how the world could have come to be populated by different groups of animals at different times. This doesn’t seem to have bothered him. His interest, after all, was not in the origin of species but in their demise.

That such extreme events had never been observed by him or any other naturalist was another indication of nature’s mutability: in the past, it had operated differently—more intensely and more savagely—than it did at present.

This wave coincided with the spread of modern humans and, increasingly, is understood to have been a result of it. In this sense, the crisis Cuvier discerned just beyond the edge of recorded history was us.

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.

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.

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 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,

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.

The problem with the record is not that slow extinctions appear abrupt. It’s that even abrupt extinctions are likely to look protracted.

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?

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.

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.

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).

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.”

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.

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.

advantage? “If evolution works the way it usually does,” Silman said, “then the extinction scenario—we don’t call it extinction, we talk about it as ‘biotic attrition,’ a nice euphemism—well, it starts to look apocalyptic.”

One of the defining features of the Anthropocene is that the world is changing in ways that compel species to move, and another is that it’s changing in ways that create barriers—roads, clear-cuts, cities—that prevent them from doing so.

One of the striking characteristics of the Anthropocene is the hash it’s made of the principles of geographic distribution. If highways, clear-cuts, and soybean plantations create islands where none before existed, global trade and global travel do the reverse: they deny even the remotest islands their remoteness. The

Anthony Ricciardi, a specialist in introduced species at McGill University, has dubbed the current reshuffling of the earth’s biota a “mass invasion event.” It is, he has written, “without precedent” in the planet’s history.

Roy van Driesche, an expert on invasive species at the University of Massachusetts, has estimated that out of every hundred potential introductions, somewhere between five and fifteen will succeed in establishing themselves. Of these five to fifteen, one will turn out to be the “bullet in the chamber.”

A species that’s been transported to a new spot, especially on a new continent, has left many of its rivals and predators behind. This shaking free of foes, which is really the shaking free of evolutionary history, is referred to as “enemy release.”

The author David Quammen cautions that while it is easy to demonize the brown tree snake, the animal is not evil; it’s just amoral and in the wrong place. What Boiga irregularis has done in Guam, he observes, “is precisely what Homo sapiens has done all over the planet: succeeded extravagantly at the expense of other species.”

From the standpoint of the world’s biota, global travel represents a radically new phenomenon and, at the same time, a replay of the very old. The drifting apart of the continents that Wegener deduced from the fossil record is now being reversed—another way in which humans are running geologic history backward and at high speed.

Emi got pregnant again in the fall of 2000. This time, Roth put her on liquid hormone supplements, which the rhino ingested in progesterone-soaked slices of bread. Finally, after a sixteen-month gestation, Emi gave birth to Andalas, a male. He was followed by Suci—the name means “sacred” in Indonesian—and then by another male, Harapan.

It would mean that the current extinction event began all the way back in the middle of the last ice age. It would mean that man was a killer—to use the term of art an “overkiller”—pretty much right from the start.

They would have had no way of knowing that centuries earlier, mammoths and diprotodons had been much more common. Alroy has described the megafauna extinction as a “geologically instantaneous ecological catastrophe too gradual to be perceived by the people who unleashed it.” It demonstrates, he has written, that humans “are capable of driving virtually any large mammal species extinct, even though they are also

Before humans emerged on the scene, being large and slow to reproduce was a highly successful strategy, and outsized creatures dominated the planet. Then, in what amounts to a geologic instant, this strategy became a loser’s game.

Though it might be nice to imagine there once was a time when man lived in harmony with nature, it’s not clear that he ever really did.

“We are brought suddenly to the realization that the universality of mankind and the love of beauty go beyond the boundary of our own species,” he wrote in a book about his discovery, Shanidar: The First Flower People.

This theory holds that all modern humans are descended from a small population that lived in Africa roughly two hundred thousand years ago. Around a hundred and twenty thousand years ago, a subset of that population migrated into the Middle East, and from there, further subsets eventually pushed northwest in Europe, east into Asia, and all the way east to Australia.

The modern humans “replaced” the archaic humans, which is a nice way of saying they drove them to extinction. This model of migration and “replacement” implies that the relationship between Neanderthals and humans should be the same for all people alive today, regardless of where they come from.

Lowland gorillas have declined even faster; it’s estimated the population has shrunk by sixty percent just in the last two decades. Causes of the crash include poaching, disease, and habitat loss; the last of these has been exacerbated by several wars, which have pushed waves of refugees into the gorillas’ limited range.

With the capacity to represent the world in signs and symbols comes the capacity to change it, which, as it happens, is also the capacity to destroy it. A tiny set of genetic variations divides us from the Neanderthals, but that has made all the difference.

DOES it have to end this way? Does the last best hope for the world’s most magnificent creatures—or, for that matter, its least magnificent ones—really lie in pools of liquid nitrogen? Having been alerted to the ways in which we’re imperiling other species, can’t we take action to protect them? Isn’t the whole point of trying to peer into the future so that, seeing dangers ahead, we can change course to avoid them?

As soon as humans started using signs and symbols to represent the natural world, they pushed beyond the limits of that world. “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.”

Pesticides are one possibility; though they’re aimed at “pest” species, the chemicals don’t discriminate between insects that damage crops and those that pollinate them. (Even protected areas in Germany may be affected by pesticides, since many of those areas abut agricultural land.)

“I was an English major,” Dyer said. “And these kinds of interactions, these stories, are like poems.” When so many are lost, it’s like burning down a library.

(The mere existence of the Ichneumonidae, Charles Darwin once argued to a friend, was enough to disprove the Biblical theory of creation, as no “beneficent and omnipotent God” would have designed such a ghoulish, murderous creature.)

So far, there’s no evidence that entire families of insects are disappearing, the authors of the Annals of the Entomological Society of America study observed. However, they went on, “if the current biodiversity crisis extinguishes diverse higher-level clades, then perhaps humans will have caused insects’ first true mass extinction.” In other words, as far as insects are concerned, humans could prove to be more dangerous than an asteroid.

And yet even these victories are far from secure. A small population is always vulnerable, and the larger forces that drove species like the Guam kingfisher and Gilbert’s potoroo to the brink are still very much at work. To alter the course of the current extinction event would require change on the scale of the Anthropocene itself.

For the sake both of ourselves and of the millions of other species with whom we share this planet, these are all steps that we should take. But I don’t see a lot of evidence that we are prepared to take them.