Human Migration From Asia To Americas

Finding A new set of ideas on migration to North America have been proposed. One idea is a hypothesis that seems to map the peopling process during the pioneering phase and well beyond, and another is that at the same time there was much more genetic diversity in the founder population than was previously believed.
The Conventional View

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. One questions is do the ancestors of Native Americans derive from only a small number of “founders” who trekked to the Americas via the Bering land bridge? Also, how did their migration to the New World proceed? And was climate change involved; did the climate have anything to do with their migration? And finally what took them so long?

Changing the Conventional View

A phylogeographic analysis of a new mitochondrial genome dataset allows scientists to draw several conclusions.

30,000 years ago - clades

First, the ancestral population on its way to the Americas paused in Beringia long enough for specific mutations to accumulate. These mutations 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. 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.)

Another way to express this is 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.

Lineages are distributed quickly not gradually
Founding lineages or haplotypes are uniformly distributed across North and South America instead of exhibiting a nested structure from north to south. So after the Beringian standstill, the initial North to South migration occured in a swift pioneering process, not a gradual diffusion.

Bi-directional Migrations to North America then Back to Beringia

The DNA data also suggest a lot more going back and forth than was previously suspected of populations during the past 30,000 years in Northeast Asia and North America. The dataset analysis 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. There was a bi-directional gene flow between Siberia and the North American Arctic.

Using Mitochondrial datasets from populations in the Americas and East Asia
The investigation of the pioneering phase in the Americas, a research 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.

What Mitochondrial DNA data reflects

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 dataset sequence was used to direct genotyping from 20 American and 26 Asian populations.

The Discovery 3 New Sub-Clades
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.

Disconnect in migration dates
Did the migration occur quickly or slowly? Migration may have occured 30,000 years ago, but the earliest archeological evidence is that it occurred only 15,000 years ago.

The point of departure places Homo sapiens at the Yana Rhinoceros Horn Site in Siberia 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.

Two possible scenarios
First 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).

The second is that 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).

The conclusion of the 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.

The 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. So they moved into the Americas quickly.