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 Neanderthal males drawn to human enclaves? Were these interactions peaceful, secretive, or violent?
“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 population genetics expert Xinjun Zhang of the University of Michigan.
But the study, published Thursday in Science, shows “that whenever Neanderthals and modern humans have mated, there has been a preference for male Neanderthals and female modern humans,” said author Alexander Platt, a genetics researcher at the University of Pennsylvania.
Scientists know the two species mated because there is a small percentage of Neanderthal DNA in most modern humans outside of sub-Saharan Africa — including genes that help us fight certain diseases. However, scientists have also known that 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 in non-sex chromosomes. Scientists previously thought these specific genes were simply not beneficial and were filtered out by evolution. Or, they thought the difference could be explained by how the two species intermingled.
To solve the riddle, Platt and colleagues looked at the Neanderthal genome and the human DNA that got interspersed during a “mating event” 250,000 years ago. When comparing these genes, they found more of a human fingerprint on the Neanderthal X chromosome — the same chromosome that, in humans, has less Neanderthal DNA than expected.
The most likely explanation for this mirror-image pattern is mating behaviour. This is due to how sex chromosomes are passed to children. Because genetic females have two X chromosomes and genetic males have one X and one Y, 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.
“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 was not involved with the study.
The study cannot totally rule out other explanations. For example, Zhang said it is possible that the offspring of human males and Neanderthal females simply did not survive as well. But the simplest and most likely explanation, the study found, is also the most interesting.
“It’s not the result of a strictly Darwinian survival of the fittest,” Platt said. “It’s really the result of how we interact with each other, and what our culture, society and behaviour are like.”
Associated Press