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9 ancient man-made environmental catastrophes

By: Bryan Nelson on Nov. 25, 2009, 8:38 a.m.
North American megafauna extinction

Photo: LadyofHats/Wiki Commons

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North American megafauna extinction

The Americas once were inhabited by magnificent beasts, some of the largest mammals ever to have walked the Earth. They included giant ground sloths (pictured here), woolly mammoths, horses, giant beavers, massive cave bears, and even American lions and cheetahs. While experts debate some of the causes of the extinctions, no one denies the eerie coincidence that all these creatures went extinct around 13,000 years ago, just as human hunters first arrived from across the Bering land bridge, wielding stone tools.