When human ancestors began scavenging for meat regularly on the open plains of Africa about 2.5 million years ago, they apparently took more than their fair share of flesh. Within a million years, most of the large carnivores in the region—from saber-toothed cats to bear-size otters—had gone extinct, leaving just a few "hypercarnivores" alive, according to a study presented here last week at a workshop on climate change and human evolution at Columbia University's Lamont-Doherty Earth Observatory.
Humans have driven thousands of species extinct over the millennia, ranging from moas—giant, flightless birds that lived in New Zealand—to most lemurs in Madagascar. But just when we began to have such a major impact is less clear. Researchers have long known that many African carnivores died out by 1.5 million years ago, and they blamed our ancestor, Homo erectus, for overhunting with its new stone tools. But few scientists thought there were enough hominins—ancestors of humans but not other apes—before that to threaten the fierce assortment of carnivores that roamed Africa, or that the crude stone tools that our ancestors began to wield 2.6 million years ago could be used for hunting. Besides, it was probably much more dangerous for the puny hominins alive then, such as Australopithecus afarensis, whose brain and body were only a bit bigger than a chimp's, to grab carcasses than it was for supersized carnivores such as giant hyenas, cats, and otters to devour hominins. "One of my favorite images is of an Au. afarensis being dragged down by a giant otter," says vertebrate paleontologist Lars Werdelin at the Swedish Museum of Natural History in Stockholm.