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Why do East Asians have 20% more Neanderthal DNA than Europeans?
Carl Zimmer | February 23, 2015
In 2010, scientists made a startling discovery about our past: About 50,000 years ago, Neanderthals interbred with the ancestors of living Europeans and Asians. Researchers also have found a peculiar pattern in non-Africans: People in China, Japan and other East Asian countries have about 20 percent more Neanderthal DNA than do Europeans.
Joshua M. Akey, a geneticist at the University of Washington, and the graduate student Benjamin Vernot recently set out to test possible explanations for the comparative abundance of Neanderthal DNA in Asians. The theory that made the most sense was that Asians inherited additional Neanderthal DNA at a later time. In this scenario, the ancestors of Asians and Europeans split, the early Asians migrated east, and there they had a second encounter with Neanderthals.
UCLA geneticist Dr Kirk E. Lohmueller and graduate student Bernard Y. Kim approached the same genetic question, but from a different direction. They constructed a computer model of Europeans and Asians, simulating their reproduction and evolution over time. After many trials, they found that a model that included a second interbreeding, another “pulse” of Neanderthal genes into the Asian population, fit the existing data the best.
But the two-pulse hypothesis also poses a puzzle of its own. If Neanderthals became extinct 40,000 years ago, they may have disappeared before Europeans and Asian populations genetically diverged. How could there have been Neanderthals left to interbreed with Asians a second time?
Higher Levels of Neanderthal Ancestry in East Asians Than in Europeans
Jeffrey D. Wall1,, Melinda A. Yang, Flora Jay, Sung K. Kim,
Eric Y. Durand, Laurie S. Stevison1, Christopher Gignoux1,
August Woerner, Michael F. Hammer and Montgomery Slatkin
Neanderthals were a group of archaic hominins that occupied most of Europe and parts of Western Asia from roughly 30-300 thousand years ago (Kya). They coexisted with modern humans during part of this time. Previous genetic analyses that compared a draft sequence of the Neanderthal genome with genomes of several modern humans concluded that Neanderthals made a small (1-4%) contribution to the gene pools of all non-African populations. This observation was consistent with a single episode of admixture from Neanderthals into the ancestors of all non-Africans when the two groups coexisted in the Middle East 50-80 Kya. We examined the relationship between Neanderthals and modern humans in greater detail by applying two complementary methods to the published draft Neanderthal genome and an expanded set of high-coverage modern human genome sequences. We find that, consistent with the recent finding of Meyer et al. (2012), Neanderthals contributed more DNA to modern East Asians than to modern Europeans. Furthermore we find that the Maasai of East Africa have a small but significant fraction of Neanderthal DNA. Because our analysis is of several genomic samples from each modern human population considered, we are able to document the extent of variation in Neanderthal ancestry within and among populations. Our results combined with those previously published show that a more complex model of admixture between Neanderthals and modern humans is necessary to account for the different levels of Neanderthal ancestry among human populations. In particular, at least some Neanderthal-modern human admixture must postdate the separation of the ancestors of modern European and modern East Asian populations.
http://www.genetics.org/content/early/2013/02/04/genetics.112.148213.short
[snip] ...the persistence through time of extra Neanderthal ancestry in southern Europe needs further study. Populations within East Asia also show differences in Neanderthal similarity. North China has a bit more Neanderthal, on average, than South China according to the samples, though all are identified as ethnic Han Chinese. In Africa, the Yoruba of Nigeria have substantially more Neanderthal similarity than the Luhya of Kenya. This is puzzling, because the geographic location of the Luhya in East Africa seems better placed for Neanderthal similarity to appear, whether through ancient population structure or through the backmigration into Africa of Neanderthal descendants. Instead, the Yoruba in West Africa are the recipients of Neanderthal genes. [/snip]
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