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.
References:
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).
No comments:
Post a Comment