DNA Study Reveals Pattern in Ancient Human-Neanderthal Relationships

New genetic research published in Science journal suggests that when humans and Neanderthals interbred tens of thousands of years ago, it was typically female humans pairing with male Neanderthals. The findings come from analyzing DNA patterns in both species' chromosomes, though the reasons behind these ancient relationships remain a mystery.

NEW YORK (AP) — When humans and Neanderthals shared the same territories thousands of years ago, they occasionally formed intimate relationships. Now, fresh genetic research is shedding light on the nature of these ancient encounters.

The latest DNA study suggests a distinct pattern: these prehistoric pairings typically involved human women and Neanderthal men, rather than the reverse combination.

The circumstances surrounding these relationships remain mysterious. Scientists wonder whether human females migrated into Neanderthal communities, or if Neanderthal males were attracted to larger human settlements. The nature of these encounters — whether they were peaceful, forced, or something else entirely — is still unknown.

“I don’t know if we’ll ever get a definitive answer to how this happened, since we can’t travel back in time,” said Xinjun Zhang, a population genetics specialist at the University of Michigan, who reviewed the research.

The research, released Thursday in Science journal, demonstrates “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,” explained study author Alexander Platt, a genetics researcher at the University of Pennsylvania.

Evidence of human-Neanderthal interbreeding exists in the DNA of contemporary people living outside sub-Saharan Africa, where a small but significant portion of Neanderthal genetic material persists. This ancient DNA influences our ability to combat certain illnesses while increasing vulnerability to others.

However, researchers have long noticed that Neanderthal genetic material isn’t spread uniformly across the human genetic blueprint.

Most notably, there’s a remarkable shortage of Neanderthal DNA on the human X chromosome — one of the genetic packages that determines biological sex — compared to the amount found on other chromosomes.

Initially, scientists theorized that genes in these regions might have been disadvantageous or even detrimental. They suggested that individuals carrying these genetic variations may have had reduced survival rates, leading natural selection to eliminate them over generations.

Alternatively, researchers considered whether the pattern might reflect the dynamics of how the two species interacted.

To investigate this puzzle, Platt’s team examined the Neanderthal genetic code and the human DNA that became mixed in during interbreeding approximately 250,000 years ago.

Their analysis revealed a greater presence of human genetic markers on the Neanderthal X chromosome — the same chromosome that shows reduced Neanderthal DNA in modern humans.

This mirror-image distribution pattern most likely stems from mating patterns, according to Platt. The explanation lies in how sex chromosomes transfer from generation to generation. Since biological females carry two X chromosomes while biological males have one X and one Y chromosome, roughly two-thirds of all X chromosomes in any population come from mothers.

If human females more frequently paired with Neanderthal males than vice versa, the genetic evidence would match exactly what researchers discovered: increased human DNA in Neanderthal X chromosomes and decreased Neanderthal DNA in human X chromosomes.

“I think that they’ve taken some really important steps in filling missing pieces to the puzzle,” said Joshua Akey, an evolutionary genomics expert at Princeton University who wasn’t part of the research team.

The study doesn’t completely eliminate alternative theories. Zhang noted that it’s conceivable that children born to human fathers and Neanderthal mothers simply had lower survival rates.

However, the most straightforward and probable explanation is also the most intriguing, according to the research. “It’s not the result of a strictly Darwinian survival of the fittest,” Platt explained. “It’s really the result of how we interact with each other, and what our culture and society and behavior is like.”