Earth on track for epic die-off, scientists say (Peter Fimrite)

A reconstructed scene in the Pleistocene of western North America, showing a group of three sabertooth cats (Smilodon fatalis). The species went extinct soon after humans arrived on the continent 13,000 years ago (Artwork by Mauricio Anton).

Dec. 19, 2009 (San Francisco Chronicle) -- If the course of human history is any model, then the wheels are already turning on Earth's sixth mass extinction, thanks to habitat destruction, pollution and now global warming, a scientific analysis of millions of years of data revealed Friday.

The study of the fossil and archaeological record over the past 30 million years by UC Berkeley and Penn State University researchers shows that between 15 and 42 percent of the mammals in North America disappeared after humans arrived.

That means North American mammals are well on the way - perhaps as much as half way - to a level of extinction comparable to other epic die-offs, like the one that wiped out the dinosaurs.

Anthony Barnosky, a UC Berkeley professor of integrative biology and co-author of the study, said the most dramatic human-caused impacts on the ecosystem have occurred in the last century.

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