In various places around the world, the fossil record shows that not long after humans arrived, large mammals—megafauna—vanished. It’s long been hypothesized that humans hunted the massive creatures to death—such would explain the suspicious timing. But could a bunch of ragtag early humans, absent modern weaponry, have really managed to kill off tens of thousands—more, even—of these large, powerful creatures?

Yes, it turns out. Megafauna’s great size came at a cost: a slow reproduction schedule, with maybe only one new offspring every other year. Such a strategy was, for a long while, very successful. Once a child made it to adulthood, it was more or less assured survival and a chance at reproduction. At least, until humans arrived.

When Alroy ran the simulations for North America, he found that even a very small initial population of humans—a hundred or so individuals—could, over the course of a millennium or two, multiply sufficiently to account for pretty much all of the extinctions on the record. This was the case even when the people were assumed to be only fair-to-middling hunters. All they had to do was pick off a mammoth or a giant ground sloth every so often, when the opportunity arose, and keep this up for several centuries. This would have been enough to drive the population of slow-reproducing species first into decline and then, eventually, all the way down to zero.

Kolbert, The Sixth Extinction, page 234

The humans responsible for these extinctions may never have even noticed what they had done.

For people involved in it, the decline of the megafauna would have been so slow as to be imperceptible. 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 capable of going to great lengths to guarantee that they do not.”

Kolbert, The Sixth Extinction, page 234

And that “geologically instantaneous catastrophe too gradual to be perceived” not only affects our perception of large mammals, but many other species as well:

Three biologists concluded…that biomass—the total weight of living things—off North America’s east coast may have declined by 97 percent since written records began. The failure of coastal residents and scientists to recognize such a shocking diminution seemed to Pauly explainable only by a long-term pattern of amnesia. Each generation of people saw the coast they grew up on as the normal state of nature and measured the declines of sea life against that baseline. With every new generation, the baseline shifted—“a gradual accommodation of the creeping disappearance,” Pauly said. We are forgetting what the world used to look like.

MacKinnon, The Once and Future World, page 19

Presumably, this is exactly what the written record ought to have prevented. By extending memory beyond our own lifespans, writing promises to help us understand time and knowledge on a longer scale. Alas, it seems more often than not the frailties of the human mind exceed our efforts to improve it.

Around thirty thousand years ago, Neanderthals vanished from Europe. Like the megafauna of the age, the Neanderthals’ decline suspiciously coincides with the arrival of modern humans. But what separated humans and Neanderthals? Why didn’t they put up more of a fight?

It is often speculated that the humans who sketched on the walls of the Grotte des Cambarelles thought their images had magical powers, and in a way they were right. The Neanderthals lived in Europe for more than a hundred thousand years and during that period they had no more impact on their surroundings than any other large vertebrate. 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.

Kolbert, The Sixth Extinction, page 258

That is, imagery and language—the ability to communicate over time—gave humans the edge over their Neanderthal brethren. The same images which we celebrate for their art were responsible for the destruction of countless species, some very much like us.

“In many ways human language is like a 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.”…If you want to think about why humans are so dangerous to other species, you can picture a poacher in Africa carrying an AK-47 or a logger in the Amazon gripping an ax, or, better still, you can picture yourself, holding a book in your lap.

Kolbert, The Sixth Extinction, page 266

Emphasis mine. In Lilith’s Brood, the alien species who arrive on Earth talk often of the “human contradiction.” They describe it as a seductive attraction, a quality that makes humans capable both of extraordinary creation and of nearly unlimited destruction. They are simultaneously drawn to it and repulsed by it. Just as we are, I suppose.