As early humans spread out across the world, their toughest challenge was colonizing the Americas because a huge ice sheet blocked the route. It has long been thought that the first Americans were Clovis people, who arrived 13,000 years ago. But an underwater discovery in Mexico suggests people arrived earlier — coming by boat, not on foot.

The migration of Homo sapiens into Asia is detailed. Recent evidence suggests the exodus from Africa began earlier than thought; and that ancient humans mated with another human species known as the Denisovans.