You are in the tundra plains of South America, 13,000 years ago.

The air in the Patagonian ravine was surprisingly quiet, thick with mist and the scent of damp earth. Your camera drone had malfunctioned, forcing you to hike down into this remote, rocky ravine to retrieve it. You were in southern Chile, near the famous Milodon Cave, but much further off the tourist trail.

You stopped, listening. The sound wasn't wind. It was a low, guttural rasp—like sandpaper rubbing against wood—mixed with a heavy, wet wheezing.

You stepped around a massive boulder and froze. Twenty feet away, two creatures were tearing at the base of a stunted tree.They were Mylodon—the giant ground sloths you had only ever seen in museums. They were massive, easily nine or ten feet long from snout to tail, their bodies thick and clumsy. Their fur was brownish-red and incredibly coarse, matted with mud and hanging in thick clumps, making them look even larger.

One of them was feeding on the leaves, standing on its hind legs, using its massive tail to tripod its weight. It used its enormous, curved claws—sharp enough to crush rock, or a predator's skull—to hook the branches and bring them down.

The second, smaller one was looking at you. Its face was bizarrely un-slothlike—it looked more like a giant, shaggy monkey than a modern tree-dwelling sloth, with a narrow snout and curious, dark eyes. It didn't possess the timid nature you expected. It gave another low, wheezing breath—a scent of musky earth and carrion hung in the air, hinting that these creatures might occasionally eat more than just plants…