S1, E1: 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. How closely related were these early Americans to today's Native Americans? It's an emotive issue, involving one of the most controversial fossils in the world, Kennewick Man.