X Chromosome Clue Suggests Neanderthal Men Often Paired With Human Women

A puzzling gap in our genes may say less about biology gone wrong and more about the social lives of ancient humans. Live Science reported that a team led by evolutionary biologist Alexander Platt has found a pattern on the X chromosome that points to a long-running mating preference between Neanderthals and modern humans.

The work offers a rare glimpse of how two kinds of humans treated each other, in a story that otherwise survives only in DNA. Here is what the research suggests.

What do we know about Neanderthals and modern humans mating?

Dozens of genetic studies have confirmed that Neanderthals and Homo sapiens had children together, across tens of thousands of years and on multiple continents, according to Live Science. Roughly 2% of the non-African genome is Neanderthal, so nearly everyone alive outside Africa carries traces of those encounters.

Yet no archaeological site shows the two groups living side by side. At Üçağızlı II, a limestone cave on Turkey's Mediterranean coast, two sets of humans left behind similar stone tools, the remains of the same prey animals and the same kinds of pierced seashell ornaments. They may have been separated by thousands of years and may not have met.

What was odd about the X chromosome?

The first high-coverage Neanderthal genome was sequenced in 2010. Over the next 15 years, researchers kept noticing that people today carry less Neanderthal DNA on the X chromosome than elsewhere in the genome. The X chromosome makes up about 5% of the DNA in cells.

The leading explanation was negative selection: the idea that Neanderthal gene variants on the X were incompatible with modern-human biology and were gradually weeded out over thousands of generations.

Why did Platt's team doubt that explanation?

If Neanderthal X variants were harmful to modern humans, then modern-human X variants should have been beneficial to Neanderthals. That would leave an excess of modern-human DNA in the parts of the Neanderthal X that matter most, such as protein-coding genes or the regulatory regions that control them.

That is not what the team found. Neanderthal X chromosomes carried more modern-human DNA in noncoding regions than in coding ones. Live Science described this as a signature of reproduction rather than of selection for better gene variants.

How can the X chromosome reveal who mated with whom?

Mothers always pass an X chromosome to their children, while fathers pass either an X or a Y. Males inherit their X only from their mother. Across a population, two-thirds of X chromosomes come from mothers and one-third from fathers. The other 22 pairs of chromosomes are contributed equally by both parents.

That imbalance makes the X a sensitive record of which sex was contributing which ancestry. If Neanderthal males paired with modern-human females more often than the reverse, more modern-human X chromosomes would flow into the Neanderthal gene pool.

What did the simulations show?

When the team ran simulations, the results suggested what Platt described as a persistent, recurring preference: males with more Neanderthal ancestry tended to reproduce with females with more modern-human ancestry. He said the trend held across at least two mating periods separated by around 200,000 years.

The researchers identified at least two distinct pulses of mixing. One, roughly 45,000 years ago, produced most of the Neanderthal DNA that lives on in non-African genomes today. The other, more than 200,000 years ago, left traces that survived in Neanderthals rather than in modern-human populations.

Why it matters

For readers, the finding changes how a familiar fact about ancestry can be read. The small share of Neanderthal DNA many people carry is not just a leftover of chance meetings; the X chromosome pattern hints at repeated, uneven pairing patterns over a very long span of time.

It also shows how much can be learned from DNA alone. Questions about whether these relationships were consensual, whether they involved violence or alliances, and who raised the children remain open, Live Science noted, because the genetic record cannot answer them directly. The result is a hypothesis based on simulations, and it describes a statistical pattern across populations rather than individual relationships.

Around Prague, based on reporting by Live Science