You are in the forested plains of South America, 2 million years ago.

The dawn was silent, the kind of silence that makes your ears ring. You were checking the perimeter of your newly acquired research plot deep in a remote, less-explored stretch of the basin, when the forest edge erupted not with a roar, but with a crack. It sounded like a massive tree snapping in half.

Through the mist and the dense palm leaves, it emerged. It wasn't a truck. It wasn't a bear. It was a 20-foot-long, four-ton disaster of shaggy, dark hair and muscle. The Megatherium walked with a slow, clumsy gait, its feet turned outward, walking on the sides of its heels to avoid digging its foot-long, inward-curling claws into the soft earth.

It stopped fifty feet away and its gaze—dark, bead-like eyes set in a narrow, cone-shaped muzzle that seemed tiny on a head that large— met yours. Slowly, almost majestically, the giant sloth reared back. It shifted its immense weight onto its massive, tripod-like tail and its thick hind legs, transforming into a 12-foot bipedal tower of fur, towering over you...