Humans and Neanderthals cozied up from time to time when they lived in the same areas tens of thousands of years ago. But we don’t know much about who got with whom, or why.

A new genetic analysis offers some ancient gossip: The pairings were more often female humans with male Neanderthals.

How exactly this happened remains a huge question mark. Did human women venture into Neanderthal populations, or were the Neanderthal males drawn to larger human enclaves? Were these interactions peaceful, confusing, secretive or even violent?

The Genetic Evidence

The study, published in the journal Science, shows “that 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,” said study author Alexander Platt, who studies genetics at the University of Pennsylvania.

Scientists know that Neanderthals and humans mated because there is a small but important percentage of Neanderthal DNA in most modern humans outside of sub-Saharan Africa — including genes that can help us fight some diseases and make us more susceptible to others.

But they have also known that the Neanderthal DNA is not distributed evenly throughout the human genome. In particular, there is a surprising lack of Neanderthal DNA in the human X chromosome compared with the amount of Neanderthal DNA in the other, non-sex chromosomes.

Mating Behavior Patterns

When comparing these genes, researchers found more of a human fingerprint on the Neanderthal X chromosome — the same chromosome that, in humans, has less Neanderthal DNA than would be expected.

The most likely explanation for this mirror image pattern is mating behavior. Because genetic females have two X chromosomes and genetic males have one X and one Y chromosome, two out of every three X chromosomes in a population are inherited from mothers.

If more human females mated with Neanderthal males than the other way around, over thousands of years you would expect to see exactly what they found: more human DNA in Neanderthal X chromosomes and less Neanderthal DNA in human X chromosomes.

Scientific Reactions

The study can’t totally rule out other explanations. For example, it’s possible that the offspring of human males and Neanderthal females just didn’t survive as well.

But the simplest and most likely explanation is also the most interesting: it reveals patterns of social interaction between our species and our ancient cousins.

Implications for Human Evolution

This research provides fascinating insights into the complex relationships between Neanderthals and modern humans. It suggests that cultural and social factors played a significant role in determining mating patterns between the two species.

The findings challenge previous assumptions about purely biological factors driving interbreeding and suggest that human behavior and social structures were important factors in these ancient interactions.

Understanding these patterns helps scientists better comprehend the genetic legacy that Neanderthals left in modern human populations and how that has influenced human evolution.