Do the ancestors of Native Americans derive from only a small number of 'founders' who trekked to the Americas via the Bering land bridge? How did their migration to the New World proceed? What, if anything, did the climate have to do with their migration? And what took them so long?
     A team of 21 researchers, led by Ripan Malhi, a geneticist in the department of anthropology at the University of Illinois, has a new set of ideas. One is a striking hypothesis that seems to map the peopling process during the pioneering phase and well beyond, and at the same time show that there was much more genetic diversity in the founder population than was previously thought.
     The new DNA analysis allowed the authors to draw several conclusions. The ancestors of Native Americans who first left Siberia for greener pastures perhaps as much as 30,000 years ago, came to a standstill on Beringia – a landmass that existed during the last glacial maximum that extended from Northeastern Siberia to Western Alaska, including the Bering land bridge – and they were isolated there long enough – as much as 15,000 years – to maturate and differentiate themselves genetically from their Asian sisters. Then, after the Beringian standstill, the initial North to South migration was likely a swift pioneering process, not a gradual diffusion.”
     The DNA data also suggest a lot more to-ing and fro-ing than has been suspected of populations during the past 30,000 years in Northeast Asia and North America. The analysis of the dataset shows that after the initial peopling of Beringia, there were a series of back migrations to Northeast Asia as well as forward migrations to the Americas from Beringia, thus 'more recent bi-directional gene flow between Siberia and the North American Arctic.'
     What puzzled the scientists originally was the disconnect between recent archaeological datings. New evidence places Homo sapiens at the Yana Rhinoceros Horn Site in Siberia – as likely a departure point for the migrants as any in the region – as early as 30,000 years before the present, but the earliest archaeological site at the southern end of South America is dated to only 15,000 years ago. "These archaeological dates suggested two likely scenarios," the authors wrote: Either the ancestors of Native Americans peopled Beringia before the Last Glacial Maximum, but remained locally isolated – likely because of ecological barriers – until entering the Americas 15,000 years before the present or the ancestors of Native Americans did not reach Beringia until just before 15,000 years before the present, and then moved continuously on into the Americas, being recently derived from a larger parent Asian population.
     Thus, for this study the team set out to test the two hypotheses: one, that Native Americans’ ancestors moved directly from Northeast Asia to the Americas; the other, that Native American ancestors were isolated from other Northeast Asian populations for a significant period of time before moving rapidly into the Americas all the way down to Tierra del Fuego. "Our data supports the second hypothesis: The ancestors of Native Americans peopled Beringia before the Last Glacial Maximum, but remained locally isolated until entering the Americas at 15,000 years before the present."

Source: Science Daily (25 October 2007)