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.