The sweep southward was marked by another drama. When Indian hunters arrived, they found the Americas teeming with big mammals that are now extinct: elephantlike mammoths and mastodonts, ground sloths weighing up to three tons, armadillolike glypt-odonts weighing up to one ton, bear-sized beavers, and sabertooth cats, plus American lions, cheetahs, camels, horses, and many others. Had those beasts survived, today's tourists in Yellowstone National Park would be watching mammoths and lions along with the bears and bison. The question of what happened at that moment of hunters-meet-beasts is still highly controversial among archaeologists and paleontologists. According to the interpretation that seems most plausible to me, the outcome was a "blitzkrieg" in which the beasts were quickly exterminated — possibly within a mere ten years at any given site. If that view is correct, it would have been the most concentrated extinction of big animals since an asteroid collision knocked off the dinosaurs sixty-five million years ago. It would also have been the first of the series of blitzkriegs that marred our supposed Golden Age of environmental innocence, and that have remained a human hallmark ever since.