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A huge horse strolls by, but you don’t pay it much attention until it rears on its hind legs by a thorn tree. Then you see that it has no hooves! Instead its feet are equipped with huge claws, which it’s using to tear down leafy branches. This beast is Moropus. 6 million years ago. However, there is a group of animals that you do recognize: chimpanzees. But they are very tall chimpanzees, and they’re walking with a very upright stance. With a shock you realize that these are Australopithecus, one of our apelike ancestors.

The boulder beneath your feet lurches. You lose your footing and tumble off, falling face first in the dusty soil. You look round to see what caused this strange tremor—and to your amazement, a door swings up in the front of the rock. Beneath it is a head with a pair of great cowlike eyes. The boulder itself was a Glyptodon curled up in its defense position. These are all typically South American animals. The North American forms are missing. You must have come to a time before the land bridge of Central America was established.

You leap to your feet, snatching a long limb bone from the soil of the cave floor. You shout, to try to distract the monster, and as your shout goes echoing down the cave, you swing the bone around your head and land it with a whack! on its nose. The bear drops to all fours and backs off. Pressing your advantage, you run forward and hit it with the bone again. With a growl it lunges back at you, then rears up once more. The great paws close around you. You feel the beginnings of the crushing hug that will grind your bones and squeeze the life out of your pulp.