Spread of early farmers may explain why Europeans have less Neanderthal ancestry than East Asians

AMERICAN ASSOCIATION FOR THE ADVANCEMENT OF SCIENCE (AAAS)—Modern humans outside of Africa can trace around 2% of their ancestry to Neanderthals, but this proportion varies geographically, with Europeans carrying slightly less of this ancestry than East Asians. Now, ancient DNA from the past 40,000 years suggests that this variation may be attributed to expansions of Neolithic farmers into Europe starting around 10,000 years ago, because these groups had less Neanderthal DNA than the hunter-gatherers who preceded them. For years, researchers have been debating when, where, and how often Neanderthals and Homo sapiens intermixed. Studies increasingly suggest that hybridization occurred in multiple waves, but it has been unclear why East Asian populations carry 8-24% more Neanderthal DNA than Western Europeans. A recent study*, authored by some researchers from the present study, proposed that spatial variations in Neanderthal ancestry could be explained by the various expansion ranges of early modern humans outside of Africa. To explore this hypothesis, Claudio S. Quilodrán and colleagues examined Neanderthal ancestry in 2,625 published human genomes from across Eurasia, ranging from about 40,000 years ago to the present day. In European DNA from around 40,000 years ago, they observed a prominent spatial gradient in Neanderthal ancestry that decreased from north to south. As hunter-gatherers continued spreading into Europe over the next 20,000 years, Neanderthal ancestry gradually declined. However, these earlier expansions still left European populations with more Neanderthal DNA than was present in East Asia during the same period. The DNA of early farmers in Europe, who started to arrive around 10,000 years ago, carried much less Neanderthal DNA than hunter-gatherers who already lived there, which may have contributed to the subsequent dilution of Neanderthal ancestry to present-day European levels. “This second range expansion is essential for explaining the pattern currently observed of lower [Neanderthal] ancestry in western Europe than in East Asia,” Quilodrán et al. write.


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