Thursday, October 25, 2007

New ideas re human migration from Asia to Americas

Questions about human migration from Asia to the Americas have perplexed anthropologists for decades, but as scenarios about the peopling of the New World come and go, the big questions have remained. 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 team’s findings appear in a recent issue of the Public Library of Science in an article titled, “Beringian Standstill and Spread of Native American Founders.”

“Our phylogeographic analysis of a new mitochondrial genome dataset allows us to draw several conclusions,” the authors wrote.

“First, before spreading across the Americas, the ancestral population paused in Beringia long enough for specific mutations to accumulate that separate the New World founder lineages from their Asian sister-clades.” (A clade is a group of mitochondrial DNAs (mtDNAs ) that share a recent common ancestor, Malhi said. Sister-clades would include two groups of mtDNAs that each share a recent common ancestor and the common ancestor for each clade is closely related.)

Or, to express this first conclusion another way, 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.

“Second, founding haplotypes or lineages are uniformly distributed across North and South America instead of exhibiting a nested structure from north to south. Thus, 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.”

To investigate the pioneering phase in the Americas, Malhi and his team, a group of geneticists from around the world, pooled their genomic datasets and then analyzed 623 complete mitochondrial DNAs (mtDNAs) from the Americas and Asia, including 20 new complete mtDNAs from the Americas and seven from Asia. The sequence data was used to direct high-resolution genotyping from 20 American and 26 Asian populations. Mitochondrial DNA, that is, DNA found in organelles, rather than in the cell nucleus, is considered to be of separate evolutionary origin, and is inherited from only one parent – the female.

The team identified three new sub-clades that incorporate nearly all of Native American haplogroup C mtDNAs – all of them widely distributed in the New World, but absent in Asia; and they defined two additional founder groups, “which differ by several mutations from the Asian-derived ancestral clades.”

What puzzled them 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 (the Beringian incubation model, BIM); 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 (direct colonization model, DCM).

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.”

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