Using ancient and modern genome-wide data, a team of researchers has found that the ancestors of all present-day Native Americans, including Athabascans and Amerindians, entered the Americas as a single migration wave from Siberia no earlier than 23,000 years ago. According to the team, this migration over a land bridge between Siberia and Alaska is distinct from the arrival of the Inuit and Eskimo, who were latecomers, spreading throughout the Artic beginning about 5,500 years ago.
The first Americans arrived in Alaska from Siberia less than 23,000 years ago. Painting by G. Mülzel.
The data consisted of the sequenced genomes of 31 living Native Americans, Siberians and people from around the Pacific Ocean, and the genomes of 23 ancient individuals from North and South America, spanning a time between 200 and 6,000 years ago.
The findings of the study confirm the most popular theory of the peopling of the Americas, but throw cold water on others, including the notion of an earlier wave of people from East Asia prior to the last glacial maximum, and the idea that multiple waves produced the major subgroups of Native Americans we see today, as opposed to diversification in the Americas.
The findings also dispel the idea that Polynesians or Europeans contributed to the genetic heritage of Native Americans.
“There is some uncertainty in the dates of the migration and the divergence between the northern and southern Amerindian populations, but as we get more ancient genomes sequenced, we will be able to put more precise dates on the times of migration,” said Dr Yun Song from the University of California, Berkeley, corresponding co-author of the study, published in the journal Science.
The researchers concluded that the northern and southern Native American populations diverged between 11,500 and 14,500 years ago, with the northern branch leading to the present day Athabascans and Amerindians broadly distributed throughout North America.
The southern branch peopled Central and South America, as well as part of northern North America.
The migration route of Siberians into North America and the subsequent split into northern and southern Amerindian populations. Analysis of current and ancient genomes shows that there also was some later interbreeding between East Asians and Inuit. Image credit: University of California, Berkeley.
“The diversification of modern Native Americans appears to have started around 13,000 years ago when the first unique Native American culture appears in the archeological record: the Clovis culture,” said study co-author Prof Rasmus Nielsen, also from the University of California, Berkeley.
“We can date this split so precisely in part because we previously have analyzed the 12,600-year-old remains of a boy associated with the Clovis culture.”
One surprise in the genetic data is that both populations of Native Americans have a small admixture of genes from East Asians and Australo-Melanesians, including Papuans, Solomon Islanders and Southeast Asian hunter gatherers.
“It’s a surprising finding and it implies that New World populations were not completely isolated from the Old World after their initial migration,” said study senior author Dr Eske Willerslev from the University of Copenhagen.
“We cannot say exactly how and when this gene flow happened, but one possibility is that it came through the Aleutian Islanders living off the coast of Alaska.”
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Maanasa Raghavan et al. Genomic evidence for the Pleistocene and recent population history of Native Americans. Science, published online July 21, 2015; doi: 10.1126/science.aab3884
