The asteroid streaks across the sky like a fiery messenger from the gods. As the sonic boom hits, small lizards scurry for cover, but some of the larger dinosaurs barely look up from their grazing. Seconds later, the asteroid heads out of the atmosphere into deep space, like a stone skipping off the surface of a pond.

It's been a close call. The incoming rock was the size of a large mountain. If it had been traveling a bit faster, or if the Earth had traveled slightly less far in its orbit, the asteroid would have hit head-on, causing utter devastation. Large creatures would have been killed almost instantly, and debris flung up into the atmosphere would have dimmed the Sun and disrupted the food chain, killing many more species. Instead, the Earth and its inhabitants shrug and continue their day.

Sixty-five million years pass. The dinosaurs diversify and continue their eons of dominance. Some learn to hunt cooperatively and invent simple social structures. Others develop metabolisms to deal with extremes of climate, and they expand their range on the planet.