Study suggests Neanderthal men more likely to pair with human women

For tens of thousands of years, humans and Neanderthals shared territories, leading to occasional intermingling between the two groups. While the fact of their co-existence and interbreeding is well-established, the specifics of these ancient encounters have largely remained a mystery – until now.

A new genetic analysis offers a fascinating glimpse into these prehistoric pairings, suggesting that the unions were more frequently between female humans and male Neanderthals. The precise nature of these interactions – whether peaceful, confusing, secretive, or even violent – continues to be a subject of speculation.

"I don’t know if we’ll ever get a definitive answer to how this happened, since we can’t travel back in time," commented population genetics expert Xinjun Zhang from the University of Michigan, reflecting on the study's implications.

Published in the journal Science, the research indicates a consistent pattern. Alexander Platt, who studies genetics at the University of Pennsylvania and co-authored the study, stated: "Whenever Neanderthals and modern humans have mated, there has been a preference for male Neanderthals and female modern humans, as opposed to the other way around."

Scientists have long known about human-Neanderthal interbreeding due to the small but significant percentage of Neanderthal DNA found in most modern humans outside of sub-Saharan Africa. This genetic legacy includes genes influencing disease resistance and susceptibility. However, it has also been noted that Neanderthal DNA is not uniformly distributed across the human genome, particularly scarce on the human X chromosome compared to other non-sex chromosomes.

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Previously, researchers theorised that genes in these locations might have been disadvantageous, leading to their evolutionary removal, or that the distribution could be explained by the dynamics of interspecies mingling.

To unravel this puzzle, Platt and his colleagues examined the Neanderthal genome, specifically looking for human DNA interspersed during a "mating event" estimated to have occurred 250,000 years ago. They discovered a stronger human genetic "fingerprint" on the Neanderthal X chromosome – precisely the chromosome that, in humans, exhibits less Neanderthal DNA than expected.

This "mirror image" pattern is most likely explained by mating behaviour, due to the way sex chromosomes are inherited. As genetic females possess two X chromosomes and genetic males one X and one Y, approximately two-thirds of X chromosomes in a population are passed down from mothers. Therefore, if more human females mated with Neanderthal males over millennia, the observed genetic pattern – more human DNA on Neanderthal X chromosomes and less Neanderthal DNA on human X chromosomes – would be the expected outcome.

Joshua Akey, an evolutionary genomics expert at Princeton University not involved in the study, praised the findings, saying: "I think that they’ve taken some really important steps in filling missing pieces to the puzzle."

While the study cannot entirely rule out other possibilities – for instance, that offspring from human males and Neanderthal females might have had lower survival rates, as Zhang noted – the most probable and intriguing explanation points to social dynamics. "It’s not the result of a strictly Darwinian survival of the fittest," Platt concluded. "It’s really the result of how we interact with each other, and what our culture and society and behaviour is like."