Gone for good. Large North American herbivores such as ground sloths perished at human hands, researchers say.

History's Greatest Serial Killers

Until the late Pleistocene era, 11,000 to 50,000 years ago, big, exotic mammals and flightless birds roamed the planet. Then, suddenly, they were gone. Who killed them? Two studies in the 8 June issue of Science finger early human hunters as the serial killers.

The mammalian die-offs were colossal. In Australia, 28 genera and 55 species of vertebrates are estimated to have vanished--including fearsome claw-footed kangaroos and the whopping 100-kilogram Genyornis, the heaviest bird ever known. Ice Age America boasted huge saber-toothed tigers, woolly bison, giant antelopes, and the woolly mammoth. Humans have long been the suspects, but analyses of past climates, computer models, and conventional archaeological and paleontological studies have all failed to provide conclusive evidence of their guilt.

Hoping to finger a culprit, a team led by geochronologist Richard Roberts of the University of Melbourne and mammalogist Timothy Flannery of the South Australian Museum dated sediments from 28 sites across Australia using optical and thorium-uranium methods. They found that large animals at the sites were buried between 51,200 and 39,800 years ago, just as human beings were spreading across Australia (Science, 8 June, p. 1888). Roberts and Miller think aborigines changed vegetation by burning the landscape, possibly to make hunting and traveling easier. The result: less food for big browsing animals, and hunting and climate fluctuations may then have tipped them to oblivion. "The bulk of the evidence is now clearly aligned with a human explanation for the [Australian] event," agrees Gifford Miller, a geochronologist at the University of Colorado, Boulder.

In North America, early hunter-gatherers may have pursued their prey south into the heart of the continent--ultimately driving populations to extinction. To test this idea, John Alroy, an evolutionary biologist at the University of California, Santa Barbara, ran a large-scale computer simulation of virtual hunters moving into virgin territory in late Pleistocene North America, starting 14,000 years ago. He found that even the slowest, clumsiest hunters caused mass extinctions (Science, 7 June, p. 1893).

Not everyone is convinced. At the American Museum of Natural History in New York City, biologists Ross MacPhee and Alex Greenwood blast Alroy's model, because they say overkill can't explain why the massive hunting stopped 10,000 years ago. Instead, he and Greenwood suspect that the huge animals succumbed to a highly contagious, lethal virus introduced by human newcomers.

Related sites

Pleistocene Mammals of the Midwestern U.S.
Paper about the extinction of the Australian megafauna
The South Australian Museum Online