As humans spread out across the world, their toughest challenge was to colonize the Americas -- because of a huge ice sheet blocking their route. It has long been thought that pioneers -- known as Clovis people -- arrived about 13,000 years ago. But an underwater discovery in Mexico is forcing the story to be re-written. How closely related were the First Americans and today's Native Americans? It's a matter of huge controversy, focused on Kennewick Man. There are very few other skeletons in the world that engender such strong feelings.