1. Americas

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 pioneers, known as Clovis people, arrived about 13,000 years ago, but an underwater discovery in Mexico suggests people arrived earlier than previously thought — and by boat, not on foot. How closely related were these First Americans to today’s Native Americans? It’s a controversial matter, focused on Kennewick Man. Few other skeletons engender such strong feelings.