Researchers have long debated what caused the mass extinction of woolly mammoths and other large animals in several parts of the world toward the end of the Pleistocene epoch. One popular theory posits that climate shifts during the last Ice Age extinguished the megafauna. But studies described today in the journal Science side with the other leading hypothesis, which holds that early humans brought about the end of these beasts.

In the first study, Australian researcher Richard G. Roberts of the University of Melbourne and his colleagues determined that Australia's megafaunal mass extinction event¿which claimed some early kangaroos and other giant marsupials, to name a few¿occurred not 20,000 years ago, as previously thought, but around 46,000 years ago. The team arrived at the new date using both radiometric and "optical" dating methods on sediments from 27 sites across the country that had contained megafauna fossils. This revised timing for the die-off, the researchers note, means that the climate factors that had supposedly done these animals in can no longer be invoked to explain their disappearance. Moreover, the team's finding places the extinction much closer to the arrival of humans to Australia, suggesting that the two events were linked. The new date alone, however, cannot reveal whether human hunting, habitat destruction or some other human factor caused the die-off.

In the second report, John Alroy of the University of California at Santa Barbara describes a computer simulation of the end-Pleistocene megafaunal extinction in North America showing that even low levels of human hunting would have driven the Ice Age behemoths out of existence. Importantly, the simulation, which assumes a slow human population growth rate and low maximum hunting efforts, correctly predicts the fate of 32 out of 41 megafaunal species. These findings, Alroy argues, show that in fact anthropogenic extinction was unavoidable. "The overkill model thus serves as a parable of resource exploitation," he concludes, "providing a clear mechanism for a geologically instantaneous ecological catastrophe that was too gradual to be perceived by the people who unleashed it."