The Sixth Extinction: An Unnatural History

by Elizabeth Kolbert

Army ants are famously voracious; a colony on the march can consume thirty thousand prey—mostly the larvae of other insects—per day.

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There’s a whole class of birds known as obligate ant-followers. These are almost always found around ant swarms, eating insects the ants have flushed out of the leaf litter. Other birds are opportunistic ant-followers and peck around the ants when, by chance, they encounter them.

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A species that needs to migrate to keep up with rising temperatures, but is trapped in a forest fragment—even a very large fragment—is a species that isn’t likely to make it. 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.

“The continents must have shifted,” he wrote. “South America must have lain alongside Africa and formed a unified block. … The two parts must then have become increasingly separated over a period of millions of years like pieces of a cracked ice floe in water.” At one time, Wegener hypothesized, all of the present-day continents had formed one giant supercontinent, Pangaea. Wegener’s theory of “continental drift,” widely derided during his lifetime, was, of course, to a large extent vindicated by the discovery of plate tectonics.

The process of remixing the world’s flora and fauna, which began slowly, along the routes of early human migration, has, in recent decades, accelerated to the point where in some parts of the world, non-native plants now outnumber native ones. During any given twenty-four-hour period, it is estimated that ten thousand different species are being moved around the world just in ballast water.

Long-term relationships between pathogens and their hosts are often characterized in military terms; the two are locked in an “evolutionary arms race,” in which, to survive, each must prevent the other from getting too far ahead. When an entirely new pathogen shows up, it’s like bringing a gun to a knife fight. Never having encountered the fungus (or virus or bacterium) before, the new host has no defenses against it. Such “novel interactions,” as they’re called, can be spectacularly deadly.

One of these is the event’s timing. The megafauna extinction, it’s now clear, did not take place all at once, as Lyell and Wallace believed it had. Rather, it occurred in pulses. The first pulse, about forty thousand years ago, took out Australia’s giants. A second pulse hit North America and South America some twenty-five thousand years later. Madagascar’s giant lemurs, pygmy hippos, and elephant birds survived all the way into the Middle Ages. New Zealand’s moas made it as far as the Renaissance.

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The sequence of the pulses and the sequence of human settlement, meanwhile, line up almost exactly. Archaeological evidence shows that people arrived first in Australia, about fifty thousand years ago. Only much later did they reach the Americas, and only many thousands of years after that did they make it to Madagascar and New Zealand.

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In a similar vein, Jared Diamond has observed: “Personally, I can’t fathom why Australia’s giants should have survived innumerable droughts in their tens of millions of years of Australian history, and then have chosen to drop dead almost simultaneously (at least on a time scale of millions of years) precisely and just coincidentally when the first humans arrived.”

Megaherbivores generate mega amounts of shit, as is clear to anyone who’s ever spent time standing behind a rhino. The ordure provides sustenance to fungi known as Sporormiella. Sporormiella spores are quite tiny—almost invisible to the naked eye—but extremely durable. They can still be identified in sediments that have been buried for tens of thousands of years. Lots of spores indicate lots of large herbivores chomping and pooping away; few or no spores suggest their absence.

“A very large mammal is living on the edge with respect to its reproductive rate,” he told me. “The gestation period of an elephant, for example, is twenty-two months. Elephants don’t have twins, and they don’t start to reproduce until they’re in their teens. So these are big, big constraints on how fast they can reproduce, even if everything is going really well. And the reason they’re able to exist at all is that when animals get to a certain size they escape from predation. They’re no longer vulnerable to being attacked. It’s a terrible strategy on the reproductive side, but it’s a great advantage on the predator-avoidance side. And that advantage completely disappears when people show up. Because no matter how big an animal is, we don’t have a constraint on what we can eat.” This

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. And so it remains today, which is why elephants and bears and big cats are in so much trouble and why Suci is one of the world’s last remaining Sumatran rhinos.

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.

Neanderthals were extremely similar to modern humans; probably they were our very closest relatives. And yet clearly they were not humans. Somewhere in our DNA must lie the key mutation (or, more probably, mutations) that set us apart—the mutations that make us the sort of creature that could wipe out its nearest relative, then dig up its bones and reassemble its genome.

The Neanderthal sequences were, as expected, very similar to human sequences. But they were more similar to some humans than to others. Specifically, Europeans and Asians shared more DNA with Neanderthals than did Africans.

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.

hobbit skeletons found on the island of Flores, in Indonesia. The hobbits, who were discovered only in 2004, are generally believed to have been diminutive archaic humans—Homo floresiensis.

modern humans must have interbred with Denisovans, too, because contemporary New Guineans carry up to six percent Denisovan DNA. (Why this is true of New Guineans but not native Siberians or Asians is unclear, but presumably has to do with patterns of human migration.)

Presumably, since they were closely related to us, both Denisovans and hobbits had a long gestation period and therefore shared the megafauna’s key vulnerability, a low reproductive rate. All that would have been required to do them in would have been a sustained downward pressure on the number of breeding adults.

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the same holds true for our next-closest kin, which is why, with the exception of humans, all the great apes today are facing oblivion. The number of chimpanzees in the wild has dropped to perhaps half of what it was fifty years ago, and the number of mountain gorillas has followed a similar trajectory. Lowland gorillas have declined even faster; it’s estimated the population has shrunk by sixty percent just in the last two decades.

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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. Sumatran orangutans are classified as “critically endangered,” meaning they’re at “extremely high risk of extinction in the wild.” In this case, the threat is more peace than violence; most of the remaining orangutans live in the province of Aceh, where a recent end to decades of political unrest has led to a surge in logging, both legal and not.

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There is every reason to believe that if humans had not arrived on the scene, the Neanderthals would be there still, along with the wild horses and the woolly rhinos. 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.

To the extent that we can identify the causes of these revolutions, they’re highly varied: glaciation in the case of the end-Ordovician extinction, global warming and changes in ocean chemistry at the end of the Permian, an asteroid impact in the final seconds of the Cretaceous. The current extinction has its own novel cause: not an asteroid or a massive volcanic eruption but “one weedy species.” As Walter Alvarez put it to me, “We’re seeing right now that a mass extinction can be caused by human beings.”

The anthropologist Richard Leakey has warned that “Homo sapiens might not only be the agent of the sixth extinction, but also risks being one of its victims.” A sign in the Hall of Biodiversity offers a quote from the Stanford ecologist Paul Ehrlich: IN PUSHING OTHER SPECIES TO EXTINCTION, HUMANITY IS BUSY SAWING OFF THE LIMB ON WHICH IT PERCHES.