As a result, Neanderthal DNA became progressively rarer in living humans. Dr. Sankararaman and his colleagues proposed that it disappeared faster in Europeans than in Asians. The early Asian population was small, the researchers suggested, and natural selection eliminates harmful genes more slowly in small groups than in large populations. Today, smaller ethnic groups, like Ashkenazi Jews and the Amish, can have unusually high rates of certain genetic disorders.

Joshua M. Akey, a geneticist at the University of Washington, and the graduate student Benjamin Vernot recently set out to test this hypothesis. They took advantage of the fact that only some parts of our genome have a strong influence on health. Other parts — so-called neutral regions — are less important.

A mutation in a neutral region won’t affect our odds of having children and therefore won't be eliminated by natural selection. If Dr. Sankararaman’s hypothesis were correct, you would expect Europeans to have lost more harmful Neanderthal DNA than neutral DNA. In fact, the scientists did not find this difference in the DNA of living Europeans.

Dr. Akey and Mr. Vernot then tested out other 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. Dr. Akey and Mr. Vernot reported their findings in the American Journal of Human Genetics.

Dr. Lohmueller and the 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. They added some Neanderthal DNA to the ancestral population and then watched as Europeans and Asian populations diverged genetically.

The scientists ran the model many times over, trying out a range of likely conditions. But no matter which variation they tried, they couldn’t find one explaining why Asians today have extra Neanderthal DNA.