Wednesday, 29 July 2015

Peopling of the New World remains contentious

New genetic studies reach differing conclusions

It is generally accepted that the humans first reached the New World by crossing the land bridge between Siberia and Alaska during the last Ice Age. However, the number of migrations and their timing has been debated for many decades.

The Paleoamerican model states that the earliest Americans or Paleoamericans were replaced by a second, separate wave of migrants from which today’s Native Americans are descended. The model is based on apparent differences in craniofacial morphology between some early fossil remains and more recent Native American. Note that this hypothetical second migration is distinct from the much later migrations responsible for around half of Aleut-Eskimo ancestry, and a tenth of Na-Dene ancestry.

Two new studies, published respectively in the journals Science and Nature, have reached opposing conclusions. Publishing in Science, Raghavan and colleagues analysed whole genomes of 31 present-day people from the New World, Siberia and Oceania, 23 ancient New World genomes and single nucleotide polymorphism genotypes from 79 present-day people from the New World and Siberia. The ancient DNA included samples from a 4,000 year-old Saqqaq individual from Greenland and the 12,600 year-old Anzick-1 (Clovis culture) individual from Montana.

They found that the ancestors of all present-day Native Americans, including Athabascans and Amerindians, entered the New World in a single migration from Siberia no earlier than 23,000 years ago and after no more than 8,000 years of isolation in Beringia. Around 13,000 years ago, these ancestral Native Americans diversified into two basal genetic branches: one that is now dispersed across North and South America and another restricted to North America. Subsequent gene flow resulted in some Native Americans sharing ancestry with present-day East Asians, including Siberians and, more distantly, Australo-Melanesians. But populations believed to be relict Paleoamericans including the Pericúes from Mexico and the Fuego-Patagonians, are not directly related to modern Australo-Melanesians, contrary to the predictions of the Paleoamerican Model.

The second study, published by Skoglund and his colleagues in Nature, featured genomic data from 63 Native Americans, who belonged to 21 diferent populations, and showed no discernable evidence of European or African ancestry. Results showed that some Amazonian Native Americans descend partly from a founding population with an ancestry more closely related to Aboriginal Australians, New Guineans and Andaman Islanders than to any present-day Eurasians or Native Americans. This genetic signature is not seen in present-day Northern and Central Native Americans, or in the Anzick-1 genome. The source population for this Australasian-related ancestry was named ‘Population Y’ after Ypykue´ra, which means ‘ancestor’ in the Tupi language family spoken by the Suruı´ and Karitiana.

The researchers suggested that Population Y had already admixed with a lineage related to First Americans by the time it reached Amazonia, and that it was the explanation for the differing craniofacial morphology noted above. However, no ancient DNA directly extracted from remains with this morphology, so the results did not prove that these people were Population Y. The absence of linkage disequilibrium in Population Y suggests that it arrived in the New World a long time ago. Furthermore, while it shows a distant genetic affinity to Andamanese, Australian and New Guinean populations, it is not particularly closely related to any of them, suggesting that its ultimate source in Eurasia no longer exists.

It is to be hoped that future ancient DNA studies provide further insight into the results of the Skoglund study.

Skoglund, P. et al., Genetic evidence for two founding populations of the Americas. Nature 525, 104-108 (2015).
Raghavan, M. et al., Genomic evidence for the Pleistocene and recent population history of Native Americans. Science 349 (6250), 841, aab3884-1-10 (2015).x

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