Native American Ancestors Came From Asia In Three Migrations
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.
In a press statement, Ruiz-Linares explains that for years there has been a debate about whether the settlement of the Americas came from one or several migrations out of Siberia.
"But our research settles this debate: Native Americans do not stem from a single migration. Our study also begins to cast light on patterns of human dispersal within the Americas," he adds.
The findings confirm what linguist Joseph Greenberg proposed in 1986. From studying language differences among Native Americans, he said the Americas must have been populated in three waves of migration.
For the study, the researchers searched more than 300,000 specific DNA markers or "snips" (SNPs, Single Nucleotide Polymorphisms) from 52 Native American and 17 Siberian groups, looking for similar and different patterns of genes.
Co-author David Reich, Professor of genetics at Harvard Medical School in the US, says they found evidence of at least three "deep lineages":
"The Asian lineage leading to First Americans is the most anciently diverged, whereas the Asian lineages that contributed some of the DNA to Eskimo-Aleut speakers and the Na-Dene-speaking Chipewyan from Canada are more closely related to present-day East Asian populations," says Reich.
It appears that 50% of the DNA of Eskimo-Aleut speakers comes from the First Americans, while in the Na-Dene-speaking Chipewyans, 90% of their DNA descends from the First Americans.
The analysis also showed that once these waves of migrations arrived in the Americas, the groups expanded southwards, hugging the coastline, splitting off along the way. After they split off, the groups mixed very little with each other, especially the ones that ended up in South America.
But while non-mixing appeared to be the general pattern after dispersal, the researchers found two striking exceptions. One shows a North-South re-mix, and the other a West-East re-mix.
In the North-South re-mix, it looks like there was some back-migration from South America northwards, and this is reflected in the genomes of Central American Chibchan-speakers, which contains DNA from two widely separated strands of Native ancestry.
In the West-East re-mix, it seems some Eskimo-Aleut speakers migrated back to Asia, as the genomes of Naukan and coastal Chukchi populations of north-eastern Siberia carry some "First American" DNA.
The analysis was not straightforward, because the researchers had to find a way to rule out genes from the European and African populations that arrived in the Americas from the late 15th century onwards.
Ruiz-Linares says they managed to develop a method to "peel back" the addition of those genes to the mix, which he says "allowed us to study the history of many more Native American populations than we could have done otherwise".
The team included researchers from: Argentina, Bolivia, Brazil, Canada, Chile, Colombia, Costa Rica, France, Guatemala, Mexico, Peru, Russia, Spain, Switzerland, the UK, and the US.
Written by Catharine Paddock PhD