Never thought about this but I decided I would put it as my first History forum post. I was surprised to find out what I did about this.

Native American Ancestors Came From Asia In Three Migrations


The ancestors of Native American populations from the tip of Chile in the south to Canada in the north, migrated from Asia in at least three waves. In what they describe as the most comprehensive survey of genetic diversity in Native Americans so far, the researchers studied variation in Native American DNA sequences. They found that while most Native American populations descend primarily from one migration, there were two later ones that also made a significant genetic contribution.

The first migration, that led to the majority of Native American populations, was of a single group called the "First Americans" that crossed from Asia to America in a land bridge called Beringia, that existed during the ice ages more than 15,000 years ago, say the researchers, whose efforts were co-ordinated by Professor Andres Ruiz-Linares of the department of Genetics, Evolution and Environment at University College London (UCL) in the UK.

The later migrants probably arrived in boats, after the land-bridge disappeared at the end of the ice ages.