Sex bias: key to a DNA puzzle Premium

Researchers found that interbreeding between Neanderthals and early modern humans favoured male Neanderthals mating with female modern humans.

For years, scientists noticed that modern humans carry very little Neanderthal DNA on their X chromosomes. To understand why, researchers from the University of Pennsylvania reversed the perspective and looked for early modern human DNA within ancient Neanderthal genomes.

In a study published in Science on February 26, they reported that interbreeding between Neanderthals and early modern humans was heavily sex-biased.

To find this, the team analysed the Altai Neanderthal genome — a female individual found in the Altai Mountains whose 1,22,000-year-old remains provide the oldest high-quality Neanderthal genome — and found a 62% excess of modern human ancestry on its X chromosome compared to its other chromosomes.

The finding allowed the team to rule out natural selection as the main cause for the missing DNA. Why? Because if biological incompatibility were the main problem, the Neanderthal X chromosome would have rejected modern human DNA. But instead it contained a surplus.

The researchers then tested whether migration patterns, such as females moving between groups, could explain the data. They found that even extreme migration models could not account for such a high 62% surplus. Instead, the most likely explanation was ancestry-specific mate preference, i.e. the researchers concluded that interbreeding primarily involved male Neanderthals and female modern humans.

Remarkably, this bias remained consistent across different encounters separated by 200,000 years, suggesting that the social dynamics between these two groups were stable over very long periods.