You are in the temperate grasslands of North America, 13,000 years ago.

The air in the spruce forest was thick, wet, and smelled of pine needles and decay. You were hiking off-trail and scouting, when the silence broke—not with a birdcall, but with a low, vibrating rumble that shook the marrow of your bones.

You froze and held your breath. Twenty yards away, standing in the middle of a shallow, marshy thicket, was a mountain of brown fur. A mastodon.

It was shorter than a mammoth, stockier, with a flat forehead and a dense, shaggy coat that looked like tangled wool. It was ripping a massive hemlock branch off a tree, its horizontal tusks—brown and weathered—slicing through the air. Behind it, a smaller, reddish-brown baby mastodon stumbled over a log.

You knew instantly you were in the wrong place. You were a ghost from a future they didn’t belong to, yet the instinct was primal. The mother (a matriarch in her small group) didn’t hesitate, as she pivoted with surprising agility on her broad feet, positioning her immense, ten-foot-tall body between you and the calf, letting out a low growl, telling you to back off...