Early modern humans repeatedly interbred with Neanderthals. Most people outside of Africa therefore carry a small proportion of Neanderthal DNA in their genome. However, there are significantly fewer Neanderthal genes on the X chromosome than on other chromosomes. A study now provides an explanation for this phenomenon. According to this, the crosses mostly took place between male Neanderthals and female Homo sapiens, so that more X chromosomes from modern humans ended up in Neanderthal populations than the other way around. The results show how prehistoric partner choice shaped our genetic makeup.
Our ancestors lived side by side with Neanderthals for many millennia. Crossing occurred again and again, so that today’s Europeans carry around one to two percent of the genetic material of our extinct sister species in their genome. But Neanderthal genes are not evenly distributed. There are large areas of the genome in which almost no Neanderthal DNA can be detected. One of these so-called “Neanderthal deserts” is located on the X chromosome. But how did this come about?
Mysterious X chromosome
“For years we simply assumed that these deserts existed because certain Neanderthal genes were biologically unfavorable to humans,” explains Alexander Platt of the University of Pennsylvania in Philadelphia. “Such disadvantageous genes would have been eliminated by natural selection during evolution.” But this explanation apparently falls short. If the genes of modern humans and Neanderthals were actually incompatible on the
However, as Platt and his team discovered, exactly the opposite is the case: analyzes of ancient DNA from Neanderthal fossils revealed that Homo sapiens genes were even more common on the Neanderthal X chromosomes than in other parts of the genome of these early humans. “We found a striking imbalance,” reports Platt’s colleague Daniel Harris. “While modern humans lack Neanderthal sequences on the X chromosomes, Neanderthals had a 62 percent excess of modern human DNA on their X chromosomes compared to their other chromosomes.”
Different mating preferences
According to the researchers, this result indicates a gender bias: “Gene flow occurred predominantly between Neanderthal men and anatomically modern human women,” explains Platt. The women passed on one X chromosome to all of their children, while the men only passed on their X chromosome to their daughters, but not to their sons. As a result, more Homo sapiens X chromosomes entered the Neanderthal gene pool than the other way around. Natural selection may have been added.
“The simplest explanation for this pattern comes from the mating preferences of early modern humans and Neanderthals,” says Platt. It is possible that Neanderthal men and Homo sapiens women found each other more attractive than when the genders were swapped. Gender-specific migration patterns could also have played a role – for example, if Neanderthal women usually stayed with their tribes while men looked for new groups. The research team hopes to uncover such possible dynamics in future genetic studies.
Source: Alexander Platt (University of Pennsylvania, Philadelphia, USA) et al., Science, doi: 10.1126/science.aea6774