For Stone Age people, reaching North and South America seems impossible - on each side vast oceans, and to the north an impenetrable ice sheet that covered the whole of Canada. So how did the first Americans get there?Dr Alice Roberts discovers evidence for an ancient corridor through the Canadian ice sheet that may have allowed those first people through. But there are problems - in particular some very ancient finds in southern Chile seem to suggest a very different way in to the Americas. Amazingly, an ancient human skull discovered in Brazil even points to an Australasian origin of the Americans. Could a route from Australia across the Pacific have been possible? A surprising answer to the problem eventually comes from a Canadian forensic scientist more used to solving murder cases.