Recent research suggests that when our species and Neanderthals interbred, it may have been primarily female Homo sapiens and male Neanderthals that mated. This conclusion was drawn from a study analyzing the genetic traces left in both populations by the intermixing.
The reasons behind this sex-biased mating pattern remain unclear. It could be that male Neanderthals favored female H. sapiens over females of their own species, or that female H. sapiens preferred Neanderthal males, or a combination of both. The study also does not provide insight into whether these matings were consensual or forced.
According to Alexander Platt from the University of Pennsylvania, the most significant finding is that these interbreeding events occurred over multiple generations. However, other geneticists like Arev Sümer from the Max Planck Institute for Evolutionary Anthropology in Leipzig, Germany, find the evidence intriguing but inconclusive, emphasizing the need for more research.
It has been known since 2010 that H. sapiens and Neanderthals interbred after modern humans migrated out of Africa into Eurasia. There were multiple periods of interbreeding, with all individuals of non-African descent today carrying some Neanderthal DNA.
The study focused on the human and Neanderthal X chromosomes, as females typically have two X chromosomes and males have one X and one Y chromosome. The researchers found that the human X chromosome is mostly devoid of Neanderthal DNA, leading to four possible explanations.
Firstly, hybrid incompatibility could have caused difficulties in health or reproduction for human-Neanderthal hybrids, but the DNA from both hominins appeared to be compatible. Secondly, natural selection favoring modern human DNA could have played a role, but this was deemed unlikely as the modern human DNA retained on the Neanderthal X chromosome was in non-functional regions.
Thirdly, a cultural explanation based on sex-based movement between societies was considered, but it did not fully account for the bias observed. This led the researchers to suggest that a mating preference – with male Neanderthals preferring female H. sapiens or vice versa – was the most plausible explanation.
However, other geneticists raise concerns about the certainty of this explanation. Sümer highlights the complete replacement of Neanderthal Y chromosomes by H. sapiens Y chromosomes in an earlier interbreeding event, suggesting a significant involvement of modern human males. Moisès Coll Macià from the Institute of Evolutionary Biology in Barcelona mentions the possibility of meiotic drive, where genetic elements cause preferential passing on of chromosomes.
In conclusion, while the study sheds light on the dynamics of interbreeding between H. sapiens and Neanderthals, further research is needed to fully understand the complexities of these ancient interactions.
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