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Ancient European admixture in the Americas, or ancient Amerindian admixture in Europe?


Learn about Y-DNA Haplogroup Q

Native American Gene Flow – Europe?, Asia and the Americas

http://dnaexplained.files.wordpress.com/2013/11/native-flow-brandt-map.png

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http://dnaexplained.files.wordpress.com/2013/11/native-flow-q-map.png

Haplogroup-Q Eupedia
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Nature is about to publish a paper on the genome of a 24,000 year old South Siberian from the Mal’ta Upper Paleolithic site near Lake Baikal. This is the oldest modern human genome sequenced to date.

Thanks to a presentation on the paper at the recent Paleoamerican Odyssey conference by the author, Eske Willerslev, we already have a fair idea about the genetic structure of this individual. Here’s a brief summary:

– Belongs to Y-DNA haplogroup R and mtDNA haplogroup U – Shows West Eurasian, South Asian and Amerindian genome-wide genetic components – Doesn’t show any signals of East Asian ancestry

Apparently, the paper also features the genome-wide ancestry results of another Upper paleolithic Siberian, from the Afontova Gora site, which are basically identical to that of the Mal’ta sample, but I can’t confirm this information at present. In any case, yesterday Science reported on Willerslev’s interpretation of the results for the Mal’ta genome:

Before 24,000 years ago, the ancestors of Native Americans and the ancestors of today’s East Asians split into distinct groups. The Mal’ta child represents a population of Native American ancestors who moved into Siberia, probably from Europe or west Asia. Then, sometime after the Mal’ta boy died, this population mixed with East Asians. The new, admixed population eventually made its way to the Americas.

Indeed, Willerslev estimates that Native Americans are about 1/3 European in terms of genome-wide genetic structure, even without any post-colonial gene flow from Europe. However, David Reich, who was quoted in the same Science article, proposed a different explanation for the results, which was obviously inspired by a recent paper he co-authored on Amerindian-like ancestry in Europe:

“Mal’ta might be a missing link, a representative of the Asian population that admixed both into Europeans and Native Americans,” Reich says.

So who’s right? Well, I’d say the truth is probably somewhere in the middle, as the old cliche goes.

I suspect the answer to this problem has been around for a while, and can be seen in the phylogeny of the human Y-chromosome. Note that the Mal’ta individual’s hapogroup, the West Eurasian-specific R, is actually more closely related to the Siberian and Amerindian-specific Q, than to other West Eurasian markers like IJK. Indeed, R and Q form a relatively unique brother clade that derives from P (Yan et al. 2013).

So it’s hardly plausible that haplogroups P, R and Q were originally specific to any biogeograhic zone(s) that we can identify today. Rather, they seem to have been markers of hunter-gatherer populations that roamed across the Mammoth Steppe all the way from Iberia to North America (see figure below from Graf 2009). These populations were probably outside the range of modern European genetic variation, and that was indeed found to be the case for their likely direct descendants from Mesolithic and Neolithic Europe (see here and here).

I don’t have a strong opinion where P split into R and Q. Perhaps in Western and Eastern Siberia, respectively? In any case, R and Q ended up dominating different parts of Eurasia, with the latter taking over a more easterly range, and obviously for that reason it was better placed to expand across the Americas. So the ancient European-like genome-wide signals picked up by Willerslev in present-day Native Americans probably arrived in the New World along with haplogroup Q, because as far as I know, there’s no evidence of the existence of pre-colonial R in the Americas.

The Ice Age played a major role in dispersing the Mammoth Steppe populations. It forced them to vacate most of Europe, Siberia and Northwest America, and seek shelter in various refugia to the south. But once the cold spell was over, they re-expanded and went on to mix with different groups in West Eurasia, Central Asia, Siberia and also the Americas, helping to form the main modern biogeographic groups in these regions. In Europe, for instance, they probably interbred with Neolithic groups genetically similar to Oetzi the Iceman to create present-day Europeans.

Perhaps this is why today different clades of Y-DNA R have such disparate regional hotspots, specifically when the more basal lineages are taken into account: R1a in Eastern Europe, R1b in the Near East and Western Europe, and R2 in South Central Asia? Indeed, perhaps this is also why when the Mal’ta individual is forced into modern genome-wide genetic clusters, he appears West Eurasian, South Asian and Amerindian? However, eventually, when enough Mammoth Steppe genomes are sequenced, we’re likely to see a new cluster emerge that is modal in these samples.

Haplogroup Q in Europe

Distribution of haplogroup Q in Europe

Götaland and Gotland in southern Sweden now have the highest frequency of haplogroup Q in Europe (5%) and almost all of it belong to the Q1a2b1 (L527) subclade.

The Romans reported that the Huns consisted of a small ruling elite and their armies comprised mostly of Germanic warriors. Gotland and Götaland is the presumed homeland of the ancient Goths. In the 1st century CE, some Goths migrated from Sweden to Poland, then in the 2nd century settled on the northern shores of the Black Sea around modern Moldova. The Huns conquered the Goths in the Pontic Steppe in the 4th century, forcing some of them to flee the Dnieper region and settled in the Eastern Roman Empire (Balkans).

It would not be improbable that some Goths and Huns moved back to southern Sweden, either before invading the Roman Empire, or after the fall of the Western Roman Empire, displaced by the Slavic migrations to Central Europe. After all, even ancient people kept the nostalgia of their ancestral homeland and knew exactly where their ancestors a few hundreds years earlier came from.

While Q1a is more Mongolian, Siberian and Native American, Q1b appears to have originated in Central Asia and migrated early to South Asia and the Middle East. The highest frequency of Q1b in Europe is found among Ashkenazi Jews (5%) and Sephardic Jews (2%), suggesting that Q1b was present in the Levant before the Jewish disapora 2,000 years ago.

Q1b is also found in Lebanon (2%), and in isolated places settled by the Phoenicians in southern Europe (Crete, Sicily, south-west Iberia). This means that Q1b must have been present in the Levant at latest around 1200 BCE, a very long time before the Hunnic migrations. One hypothesis is that Q1b reached the Middle East alongside haplogroup R1a-Z93 with the Indo-Iranian migrations from Central Asia during the Late Bronze Age.

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