What Killed America's Behemoths?

The disappearance of mammoths and giant sloths has defied explanation since the turn of the 19th century, when Thomas Jefferson sent Lewis and Clark on an expedition to, among other things, find the lost megafauna in the heart of America. Many researchers have blamed the extinction of most large mammals on the hunting excesses of nomadic humans. But could a changing climate be the culprit? In Science Smackdown, we let experts argue both sides.