Traditionally, Native Americans are believed to have descended from prehistoric hunters who walked from northeast Asia across a land bridge, formed at the end of the Ice Age, to Alaska some 12,000 years ago. American Indians resemble the people of Mongolia, China and Siberia.
However recent research indicates that the initial settlement of the continent may have instead been driven by Southeast Asians who occupied Australia 60,000 years ago and then expanded into the Americas about 13,500 years ago, prior to Mongoloid people arriving from northeast Asia.
Source: Stefan Lovgren, National Geographic News. September 3, 2003