People living in East Asia have inherited about 20 percent more Neanderthal DNA than Europeans. Some say that’s because the Neanderthal lineage was diluted in Europeans because they bred more with Africans, who lack Neanderthal ancestry for the most part. Well, not so, according to two studies published in the American Journal of Human Genetics. The differences were due to multiple occasions where Neanderthals and early East Asians mixed. 

There are several ideas for why there’s a greater proportion of Neanderthal ancestry in East Asians than in Europeans. Most of them assume that there was one interbreeding event between the Neanderthals living in Europe and parts of Asia and a population that’s ancestral to all anatomically modern, non-Africans. In addition to this “single pulse of admixture,” one hypothesis further suggests that Neanderthal DNA may have been weakly deleterious in humans. And even though natural selection weeds out the harmful stuff, Science News explains, East Asians had a smaller founding population (than Europeans), making selection less effective at removing the detrimental copies of genes.

Using a series of computer simulations, UCLA’s Bernard Kim and Kirk Lohmueller tested that hypothesis and found that the reduced efficiency of “purifying selection” in East Asians can’t explain the observed increase in the proportion of Neanderthal ancestry. In a separate study, Benjamin Vernot and Joshua Akey from the University of Washington analyzed patterns in the whole-genome data of 379 Europeans and 286 East Asians. They also found that differences in selection can’t explain the differences in Neanderthal ancestry based on a simple one-pulse model. 

In the end, both teams found that the most likely explanation is that the ancestors of modern East Asians interbred with Neanderthals more than once: There must have been a second pulse of Neanderthal gene flow. “The key idea is that there would have to have been some additional interbreeding events involving East Asians, but not Europeans,” Lohmueller tells the Daily Mail. 

However, because our species has been constantly migrating, it’s hard to pinpoint where these interactions with Neanderthals happened. “It's possible, for example, that all of the interbreeding with Neanderthals occurred in the Middle East, before the ancestors of modern non-Africans spread out across Eurasia,” Vernot says. That ancestral group would later split into Europeans and East Asians. “Shortly after they split up,” he adds, “the ancestors of East Asians interbred with Neanderthals just a little bit more.”