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.

Werdelin decided to investigate to discover when the decline began and what types of species went extinct by analyzing the jaws, teeth, snouts, and other anatomy of large and small carnivores. These features reflect the breadth of the animals' diets and the niche they occupied in the community of carnivores—an approach known as community ecology analysis. For example, many meat eaters have teeth that are specialized cutting blades, whereas omnivores that also eat fruit have crushing teeth. After comparing fossils of 78 species of carnivores that lived during five different periods of time between 3.5 million years ago (when large carnivores were at their peak) and 1.5 million years ago, Werdelin found that all but six of 29 species of large carnivores (animals that weighed more than 21.5 kilos) had gone extinct in that time. Moreover, the mass extinction began just before H. erectus appeared in the fossil record 1.9 million years ago. He also found that the community of carnivores alive 2.5 million to 2 million years ago ate a much broader range of food—with species within a community filling a wider range of dietary niches. By 1.5 million years ago, just hypercarnivores that ate only meat, such as lions and leopards, had survived while omnivores that scavenged and ate a wider range of foods, like civets, had disappeared. "Even I was surprised by the dramatic drop," Werdelin says.