Why did the paleo indians cross the land bridge?

There were only two ways to move, walk or take the boat. They probably did both (show possible routes on National Geographic [2000] map, Peopling of the Americas) . The typical explanation for the earliest humans' entrance into the New World is that people crossed from northeast Asia, Siberia, into North America, during the Pleistocene when sea levels were lower and there was a land bridge; they kept walking and made it to South America. This year the coastal hypothesis is back in favor, the idea that the earliest peoples made their way slowly down sheltered coasts of both North and South America. Though sea level has risen and no doubt drowned just about all the evidence, there are a few North American sites supporting this hypothesis. At any rate, they must have come through North America first, to a greater or lesser degree, not across the wide Pacific from Japan, even though many of the South American dates are earlier than the earliest ones from North America. Bruhns (p. 43) says they must have moved into South America down the mountain chains because those environments support large herbivores. This has been our stereotyped view of the first Americans, Big-Game Hunters extraordinaire, very macho, with big chipped stone points and lots of meat. But we need to examine this model closely.