Forty thousand years after Neanderthals became extinct, they still live on in modern humans, like a DNA trace. People outside Africa carry 1 to 2 percent Neandertal DNA in their genome. Outside Africa, because that mixing occurred during the long migration Out of Africa that one Homo sapiens definitively spread throughout the world between 70,000 and 20,000 years ago.
Geneticists have now calculated that this genetic mixing between people (Homo sapiens) and Neanderthals probably came about because Neanderthal men in particular had sex with Sapiens women. As they put it, there seems to be a ‘mating preference’ (breeding preference) to exist between Sapiens women and Neander men. The three American geneticists led by Alexander Platt (University of Pennsylvania) publish their conclusions this week Science.
But how did sexual contact between Neanderthal men and Sapiens women take place around fifty thousand years ago? Was it incidental sex or did the ‘gene transfer’ take place in the context of structural exchange? Did Sapien women marry off, or did Neanderthal men join human groups? Nobody knows. Into the research Science is purely genetic, science can only guess at the behavior behind it. At most, it has sometimes been calculated, by the German paleogeneticist Johann Krause, that the ‘basic Out of Africa population’ (from which the current non-African population descends) must have consisted of approximately 5,000 fertile individuals. And it would have somehow mixed with about 200 Neanderthals somewhere in the Middle East, 50,500 to 43,500 years ago.
How did Neanderthals and Sapiens live?
Archaeologist Wil Roebroeks is fascinated by the new research, but it remains an isolated issue, he explains in an email. The assumption that there was a preference for mating between Neanderthal men and Sapiens women simply cannot be tested with the archaeological knowledge of how Neanderthals and Sapiens lived in prehistoric times. “We know far too little about the social and demographic context of all this, we cannot support any scenario about how things could have gone between those men and women.”
To know whether it was men or women who left the group to find partners in Neanderthals and early modern humans, “you need, for example, very large data sets, with enough fossils of men and women on which you can do DNA research, and with which you can use chemical analysis to find out where they once grew up. But we don’t have those data sets at all.” The DNA research Science has now been performed on mainly three Neanderthal genomes, from Altai (Siberia, 120,000 years old), nearby Chagyrskaya (80,000 years) and Vindija (modern Croatia, 52,000 years old). There are now a total of eight Neanderthal genomes sequenced.
Alexander Platt and his colleagues Sarah Tiskhoff and Daniel Harris came up with their theory when they were looking for an explanation for the fact that Neanderthal DNA is very rare on the X chromosome in modern humans.
The ‘Neanderthal desert’ on the X chromosome is often explained by natural selection. In those places, the Neanderthal DNA could have had negative consequences and therefore disappeared quickly. But that explanation was never certain, and due to the lack of Neanderthal genomes from after the mixing, it is also unknown how the genetic mixing turned out in Neanderthals. To investigate this, they went back to previous genetic contacts between Neanderthals and humans, traces of which can be found in Neanderthal genomes.
The problem, of course, is that we have almost no genomes of Neanderthals
Because the possibly seven thousand year long period of sexual contact approximately 50,000 years ago was not the first time. Traces of contact have also been found in Neanderthal genomes Homo sapiens in the period 250,000 to 200,000 years ago, and also from the period 120,000 to 100,000 years ago. That first contact may have taken place in Europe, during a very early (temporary) migration of early Sapiens groups from Africa. The second probably took place in the Middle East.
And the surprise is: those early contacts actually led to an excess of Sapiens DNA on the X chromosome of Neanderthals. Then there could be no genetic incompatibility in that area, according to the researchers, and the selection argument therefore lapses. The excess of human DNA on the Neander X chromosome turned out to be very large, 60 percent more than on the other chromosomes. So big, in fact, that only a specific mating preference between Neanderthal males and Sapiens females can explain the size of that Sapiens intrusion on the X chromosome. Conversely, this mating preference is also the explanation for the mirror effect: the Neanderthal desert on the Sapiens X chromosome. All arising from the fact that men pass on one X chromosome to their offspring, and women pass on two.
Roebroeks is struck by how cautiously the authors formulate their conclusions: “Although they posit mating preference as the most likely explanation, they do not rule out that all kinds of demographic processes also played an important role. There also remains a role for natural selection.”
And to complicate matters further: out previous research It has been known for six years that Neanderthals were left with an essentially sapiens Y chromosome from that first mixture around 250,000 years ago. That is only possible if early modern Sapiens men had sex relatively often with Neanderthal women. Roebroeks: “The new research also mentions the possibility of these contacts. In any case, there must also be ‘some interbreeding‘, there has been some sexual contact between Neanderthal women and Sapiens men, they write. The problem, of course, is that we have almost no genomes of Neanderthal men, most of them are from women. Here too, the numbers are extremely small.”
