We already knew with certainty that Sapiens and Neanderthals had met, and that the two species had mixed. A new genetic study shows that mating took place mostly between Neanderthal males and Homo sapiens females, which could finally explain why certain areas of our genome, particularly on the X chromosome, are almost devoid of Neanderthal DNA.
For more than twenty years, geneticists have stumbled upon these “Neanderthal deserts”: regions of our genome where we find almost no sequences inherited from our extinct cousins, while most non-Africans carry on average 2% Neanderthal DNA and certain African populations up to 1.5%. There we saw traces of Neanderthal genes deemed “toxic” for our species, gradually eliminated by natural selection, but we may have been wrong, explains an article in Live Science.
Alexander Platt, a population geneticist at the University of Pennsylvania, summed up this consensus this way: “For years we simply assumed that these deserts existed because certain Neanderthal genes were biologically ‘toxic’ to humans, as is often the case when species diverge.” But in the article published on February 26 in Science, he and his colleagues defend another explanation, considered more plausible: mate preference, a well-known mechanism of sexual selection.
To test this idea, the team compared the genomes of three Neanderthals with those of 73 women from three current African populations lacking Neanderthal ancestry. Surprise: Neanderthal X chromosomes contain about 1.6 times more modern human DNA than their other chromosomes. If genetic incompatibility were the only cause, we should see the opposite: few sapiens genes among Neanderthals on this same X chromosome.
Who prefers who?
The researchers then simulated different scenarios (sex-biased migrations, natural selection, etc.). Even assuming that almost all modern humans who arrived in Neanderthal territory were women, we only obtained an excess of 1.3 at best, insufficient to explain the data. The configuration that fits best is the one where Neanderthal males reproduced more often with sapiens females than the other way around, which brings a lot of modern DNA into the Neanderthal Xs, but little Neanderthal DNA into the human Xs.
“This explanation is anchored around the possession of X chromosomes”summarizes Alexander Platt: we received relatively few Neanderthal X chromosomes, while they accumulated a lot of sapiens ancestry on their “I have no idea what preference that translates here.” Clearly, it is impossible to say whether it was Homo sapiens women who found Neanderthals more desirable, Neanderthal males who favored modern partners… or both.
Desire, opportunity, domination
Previous work on the Y chromosome, that of males, already showed the existence of crosses between Homo sapiens men and Neanderthal women. The new study therefore does not exclude these unions, but suggests that they were less frequent. “It is clear from this new study that, in fact, Neanderthal men and Homo sapiens women liked each other more than Neanderthal women and Homo sapiens men”summarize the authors. A dizzying question remains: were these contacts desired, tolerated, or forced? Genomes can’t tell the whole story.
Alexander Platt and his colleagues do not rule out more complex scenarios, combining natural selection, sex-differentiated migrations and reproductive biases. They especially emphasize the gray areas: “We just don’t have a genetic signature yet to go beyond that.”concedes the geneticist. The distribution of Neanderthal DNA in our genome is not uniform and other mechanisms could also contribute to the rarity of these sequences on our X chromosome.
To go further, the team wants to tackle another piece of the puzzle: the social organization of the two groups. Anthropologists know that mate choice is partly learned, shaped by gender norms and roles within a society. “We plan to study the evolution of social structures and gender roles among Neanderthals, which could shed light on the questionexplains Alexander Platt, but we are still far from that.”
These 2% of Neanderthal DNA in us are not just the abstract relics of ancient crossbreeding, but the trace of very concrete preferences between men and women of two different species. Desire, opportunity, domination, have shaped choices or non-choices, the imprint of which can still be read today in our genome.
