But life on Earth is not completely destroyed. Out of the ruins, nature starts over. A group of animals that have long been limited in success by the dinosaurs now become the new rulers of the Earth: mammals. Eventually, they multiply and diversify. 10,000 species explode across the planet. And one species, humans, branches off from its relatives and comes down from the trees. They walk on two legs, evolve bigger brains, and eventually, they rule the planet. They build cities that touch the sky, vehicles that can leave the planet, and weapons that can destroy it.