When Neanderthals and ancient modern humans interbred, the pairings were mostly between male Neanderthals and female humans. This finding helps explain why Neanderthal ancestry present in most humans is unevenly distributed. Anatomically modern humans carry low levels of Neanderthal ancestry, but it is not evenly shared. When the genomes of Neanderthals and modern humans are compared, striking gaps known as "Neanderthal deserts" are revealed. These are large stretches of DNA in modern humans where Neanderthal genetic contributions are unusually rare. Such deserts appear across several chromosomes and are especially prominent on the X chromosome. According to Alexander Platt and colleagues, this pattern can be explained in one of two ways: Neanderthal variants on the X chromosome were disadvantageous in modern humans and were gradually eliminated by natural selection, or early interbreeding could have occurred primarily between male Neanderthals and female modern humans, resulting in little Neanderthal X-chromosome DNA ever entering the human gene pool.
To resolve these competing theories, Platt et al. evaluated the fate of early modern human DNA that entered Neanderthal populations during an earlier episode of human-Neandertal interbreeding. By examining these Neanderthal genomes alongside genetic data from Specific sub-Saharan African populations that lack Neanderthal ancestry, the authors were able to trace patterns of ancient gene flow. Platt et al.'s analysis revealed a 62% relative excess of modern human ancestry in Neanderthal X chromosomes, suggesting that interbreeding was predominantly between male Neanderthals and female modern humans. Further modeling indicates that mate preference offers the simplest explanation for this sex bias than population migration alone, although the authors cannot exclude the possibility of demographic sex biases playing an important role. Furthermore, differential migration and mate preference may all have been at play simultaneously. Subsequent negative selection likely further reduced Neanderthal variants in functional X-linked regions.
