NEW YORK (AP) — When humans and Neanderthals lived in the same area tens of thousands of years ago, they occasionally got along. But we don’t quite know who’s with whom, or why.
A new genetic analysis offers some ancient gossip: These pairs were usually female humans with male Neanderthals.
How exactly this happened remains a huge question mark. Did human females venture into Neanderthal populations, or were Neanderthal males attracted to larger human enclaves? Are these interactions peaceful, confusing, clandestine, or violent?
“I don’t know if we can get a clear answer about how this happened because we can’t go back in time,” said Xinjun Zhang, a population genetics expert at the University of Michigan, commenting on the new analysis.
But the study, published Thursday in the journal Science, shows that “whenever Neanderthals and modern humans mate, there is a preference for male Neanderthals and female modern humans, not the other way around,” said author Alexander Platt, who studies genetics at the University of Pennsylvania.
Scientists know that Neanderthals mated with humans because most modern humans outside sub-Saharan Africa contain a small but significant amount of Neanderthal DNA, including genes that can help us fight some diseases and make us more susceptible to others.
But they also know that Neanderthal DNA is not evenly distributed throughout the human genome.
In particular, there is a striking lack of Neanderthal DNA in the human X chromosome (one of the gene bundles called sex chromosomes in every cell) compared to the amount of Neanderthal DNA in other non-sex chromosomes in the cell.
Scientists believe that perhaps genes in these locations are not beneficial at all, or even harmful. Perhaps people with these gene patterns also couldn’t survive, so over time these genes were filtered out by evolution.
Or, they thought, maybe the difference could be explained by how the two species mixed.
To try to unravel the mystery, Platt and colleagues turned to Neanderthal genomes and human DNA scattered during a “mating event” 250,000 years ago.
When comparing the genes, they found more human fingerprints on the Neanderthal X chromosome—less Neanderthal DNA than expected on the same chromosome in humans.
The most likely explanation for this mirroring pattern is mating behavior. Platt explains that this is because of the way sex chromosomes are passed from parent to child. Since genetic females have two X chromosomes and genetic males have one X and one Y chromosome, on average two of every three X chromosomes in the population are inherited from people’s mothers.
If more human females mated with Neanderthal males rather than the other way around, then a few thousand years from now you’d expect to see what they found: more human DNA in the Neanderthal X chromosome and less Neanderthal DNA in the human X chromosome.
“I think they’ve taken some very important steps to fill in the missing pieces of the puzzle,” said Joshua Akey, who studies evolutionary genomics at Princeton University and was not involved in the new study.
This study cannot completely rule out alternative explanations. The offspring of a human male and a Neanderthal female, for example, probably wouldn’t survive either, Zhang said.
But the study found that the simplest and most likely explanation was also the most interesting: “It’s not strictly Darwinian survival of the fittest,” Platt said. “It’s really a result of the way we interact with each other and our culture, society and behaviour.”
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