An article published in the latest issue of Science (Alexander Platt et al., Interbreeding between Neanderthals and modern humans was strongly sex biased. Science 391, 922-925 (2026). DOI:10.1126/science.aea6774) suggests that male Neanderthals had a soft spot for female Anatomically Modern Humans (AMH), or that AMH women chose male Neanderthals as mates, or both.
Thos of us who trace our ancestry to a non-Sub-Saharan African ancestor, carry snips of Neanderthal DNA in our chromosomes. These come from an admixture (mating) episode that took place some 45,000 to 49,000 years ago. A few thousand years later, the Neanderthals vanished and our human ancestors replaced them. But these Neanderthal genes are not uniformly distributed across our chromosomes. Why?
The Neander men mated with Human women!
The authors came to this conclusion (human women mating with Neanderhtal men) after looking at the distribution of Neanderthal alleles in our human genome. Although our ancestors mated with Neanderthals, and all humans living outside of Sub-Saharan Africa carry Neanderthal genes, there are vast swaths of autosomal genes in our 22 chromosomes that are devoid of Neanderthal alleles. An interesting anomaly is that the X chromosome us even more depleted of Neanderthal alleles (we carry another four chromosomes, either two "X" or an "X" and a "Y" if we are women or men, respectively). Why would the "X" chromosomes carry less alleles than the others? Well, this paper says that it reflects how Neanderthals contributed to our ancestry, the X chromosomes don't show Neanderthal ancestry because they are mainly human ones, mostly from human women and much less from Neanderthal women. Women contribute their X to all offspring, men do so only to their daughters, as their sons carry their Y chromosome. There could also be another factor that added to this distortion, natural selection that selected against the Neanderthal alleles.
The paper begins by stating in its Abstract that "By observing a 62% relative excess of AMH ancestry in Neanderthal X chromosomes, we characterized the interbreeding between the two groups as predominantly male Neanderthals with female AMHs" They suggest two explanations for this skew, to explain the "Neanderthal deserts across the modern human X chromosomes: (i) The lack of Neanderthal loci amongst the X chromosomes in the mode, or (ii) the contribution of Neanderthal X chromosomes was reduced from the very beginning and represents an original interbreeding that was biased toward male Neanderthals and female anatomically modern humans (AMHs)."
To validate which hypothesis is correct, they looked back, at the first admixture between AMH and Neanderthals, that took place 250 ky ago, and observed the fate of Neanderthal alleles in their offspring and applied these conclusions to the more recent admixture that ocurred 50 kya.
So they analyzed the genes of Neanderthals who lived after the AMH-Neanderthal admixture, including the Altai Neanderthal (122 kya), the two females from Chagyrskaya (80 kya) and and Vindija (52 kya). The authors concluded "that the lack of Neanderthal alleles in the modern human gene pool is not simply the result of excess incompatibility loci on the X chromosome". They also discarded genetic drift due to a small population size for the Neanderthals. Regarding the effects of Natural Selection, they found that "the Neanderthal X chromosomes did not suffer from sufficient mutational load to cause them to be generally deleterious compared with AMH X chromosomes." So evene if natural selection acted, it "was not likely sufficient to drive the broad lack of Neanderthal ancestry in the modern human X chromosome gene pool"
Caveman Courtship
Then they looked at dispersals and relocations between human and Neanderthal populations due to pair-coupling and mating (where female AMHs relocate into their Neanderthal partner's community) and modeled different scenarios. For instance, if all the AMH women of an exclusive all-female migration, mated with Neanderthal men, the excess ratio of AMH ancestry in the X chromosome of the hybrid offspring would be 1.33 (or +33%), but they observe a 1.62 excess ratio (+62%). One also has to consider the opposite effect of AMH males mating with Neanderthal women, which would lower the AMH presence in the X chromosomes of their hybrid offspring.
Since this can't raise the proportion of AMH presence in X chromosomes, they reasoned that the alternative was "mating preference" and describe it as follows: "the patterns that we observed in AMH-Neanderthal divergence and hybridization follow traditional processes of speciation and were likely colored by a persistent preference for pairings between males of predominantly Neanderthal ancestry and females of predominantly AMH ancestry over the reverse. The bias that we inferred seems to have remained consistent across admixture events separated by 200,000 years. Although we do not know what drove the biases in either event, the potential for preferences in mate choice to persist across time and space have been documented in both human and animal studies." So Neanderthal men liked AMH women more (or viceversa, or both!)
The same issue of Science has a commentary on this paper (online here) and its title is suggestive: "Surprising partner preference found in matings between Neanderthals and modern humans. Male Neanderthals tended to pair up with female modern humans, but whether intercourse was consensual is unclear." It describes the article and interviews some scholars about it, concluding that "The mating bias Tishkoff and her co-authors have uncovered reflects something about the cultures and social behaviors of both species, she says. The team did not venture to guess whether the intercourse was consensual or coerced. But to Steven Churchill, a Duke University paleoanthropologist who was not involved with the research, the finding implies aggression. If males from one species monopolized females from the other, he says, “it’s hard to reconcile that with anything but a competitive, unfriendly interaction.”"
Closing Comments
I don't see why these interactions had to be violent, but given apes and humans tendency to be sexual, and aggressive it is an option, or perhaps wokism trying to explain ancient mating behavior.
Back in September 2011, I posted on the admixture of Neanderthal males and Human females and tried to answer the question of why there is no Neanderthal (NH) mtDNA in Homo sapiens (HS) (there should be if a matrilineal lineage had survived until now, with an original Neanderthal female passing her mtDNA across the generations until the present). I reasoned that: "Since nowadays there is no Neanderthal mtDNA in HS, we can conclude that mating between HS men and NH women (was as limited) or, if more frequent, it did not lead to a continuous lineage of hybrid N/HS women... if Neanderthal men got human women pregnant, they would not pass on any NH mtDNA (the children would have their human mom’s mtDNA). But, the Neander-Dad would pass on his nuclear DNA . So, Man (NH) and Woman (HS) would be a viable route to get Neanderthal DNA into our Homo sapiens cells. This explains why we have Neander DNA (autosome chromosomes)."
However, my reasoning did not lead to a higher prevalence of Human X chromosomes in the case of Neander-Human matings! I believe thtat this is so because I took into account Haldane's Law of sterile male offspring. As you can see in the image further up, if you eliminate the boys, who each carry an X in their XY sexual chromosomes, the two girls, would make it a 50-50 proportion as each carry a Neanderthal X and a Human X. In the same ratio! Maybe Haldane's Law does not apply?
Haldane's Law or Rule "JBS Haldane noted that when hybrid crosses affect one sex more significantly than the other, it is almost always the heterogametic sex that is so affected1. Thus, the risk is greatest for XY male hybrids in mammals... A century of observations has confirmed this rule." (Source).
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