Colors of the arrows show their timing. The initial peopling of Berinigia
(in light yellow) was followed by a standstill. Then the ancestors of
Native Americans spread swiftly all over the New World. More recent
migration (shown in green) shows back-migration into Siberia and the
spread of D2a into north-eastern America.
Source: Image courtesy Ripan Mahli
Alaska may be the cradle of the New World.
A new study suggests that ancestors of the very first Americans may have cooled their heels during the height of the ice age in the Beringian steppes of western Alaska and northeastern Siberia, remaining in the Far North for thousands of years before suddenly trekking south to seize the continent from woolly mammoths and bison.
These prehistoric Alaskans could have been in Beringia as early as 30,000 years ago — long before the continental ice sheets melted — subsisting in a long-gone ecosystem of fabulous giant mammals. They also carried a surprising amount of diversity in their genes.
But once these prehistoric Beringians decided to move south, they did so with a vengeance, migrating all the way to the tip of South America within a few generations.
“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,” explains an online story from the University of Illinois, “and they were isolated there long enough — as much as 15,000 years — to maturate and differentiate themselves genetically from their Asian sisters.
“After the Beringian standstill, the initial North to South migration was likely a swift pioneering process, not a gradual diffusion.”
These new genetic findings are the latest insights into an anthropological mystery: Who were America’s first residents? Where did they come from? How and when did they get here?
For a generation, anthropologists taught that spear-chucking Asian migrants trudged across the Bering Land bridge about 12,000 years ago, when ice sheets melted and opened a corridor into the heart of North America. But during the past few decades, archaeological and genetic evidence has turned that theory inside out. Artifacts have been found at Monte Verde in Chile, for instance, that date to nearly 13,000 years ago, before the glaciers melted. In Siberia, traces of human occupation date to 27,000 years ago, before the glaciers reached their peak.
Some anthropologists now argue the first Americans were kayakers and beachcombers, moving south along the shore. There are other theories, too, suggesting that many different groups migrated to America over time.
However they traveled, it now appears that these pioneers remained in Alaskan and Siberian Beringia for eons before moving on.
Other findings: A second wave of colonizers swept out of Asia thousands of years later to colonize the Arctic. Meanwhile, some of the new Americans apparently didn’t like the local weather and migrated back into Asia. Traces of their genes can be found in native Siberians today.
Led by geneticist Ripan Malhi at the University of Illinois, the scientists analyzed 601 Native American people from 20 populations and compared their genetic fingerprint with 3764 samples from 26 Asian populations. They laid out the results in Beringian Standstill and Spread of Native American Founders, published online by the Public Library of Science.
Here is more detail from the UI online story:
CHAMPAIGN, Ill. — 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 Malhi, 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.
“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.”
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.”
Source: Czech illustrator Zdenek Burian
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.”