Determining whether the first arrival of humans or the warm-up of the American continent at the end of the last Ice Age was responsible for the demise of prehistoric sloths has puzzled scientists because both events occurred at the same time, about 11,000 years ago. But by using radiocarbon to date fossils from Cuba and Hispaniola, where humans appeared later than on the North American continent, long after the last Ice Age occurred, UF ornithologist David Steadman was able to separate the two events.

He and his colleagues found the last record of West Indian ground sloths coincided with the arrival of humans 4,400 years ago. The results are published in a Proceedings of the National Academy of Sciences paper this week.

"If climate were the major factor driving the extinction of ground sloths, you would expect the extinctions to occur at about the same time on both the islands and the continent since climate change is a global event," Steadman said.

Gary Haynes, anthropology professor at the University of Nevada, Reno, said Steadman's study "clearly shows that ground sloth extinctions in the New World didn't happen after serious changes in climate or vegetation and that the first appearance of humans must have been the decisive factor."

The fossil record shows the people who arrived in North America were making sophisticated tools out of stone, bone and ivory, Steadman said. These "big-game hunters" had a traumatic effect on the animals living there, he said.

More than three-fourths of the large species of mammals that roamed the North American landscape became extinct within a few thousand years, which, besides ground sloths, included mammoths
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Contact: David Steadman
dws@flmnh.ufl.edu
352-392-1721
University of Florida
3-Aug-2005