Scientists have known for a while that Neanderthals occasionally mated with humans eons ago. But a study in Science adds a new layer of knowledge to the subject: It turns out that much of this mating took place between Neanderthal males and female humans, "as opposed to the other way around," study author Alexander Platt of the University of Pennsylvania tells the AP. 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?
"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 with the University of Michigan. 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. 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, one of the bundles of genes in each cell known as a sex chromosome, compared with the amount of Neanderthal DNA in the other, non-sex chromosomes in the cell.
Scientists thought that maybe the genes in those locations were simply not beneficial—or even harmful. Perhaps people with those gene patterns didn't survive as well so those genes were filtered out by evolution over time. Or, they thought, maybe the difference could be explained by how the two species intermingled. To try to solve the riddle, Platt and colleagues looked instead 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 would be expected.
The most likely explanation for this mirror image pattern is mating behavior. That's because of the way sex chromosomes are passed from parents to children, explained Platt. Because genetic females have two X chromosomes and genetic males have one X and one Y chromosomes, two out of every three X chromosomes in a population, on average, are inherited from people's mothers. If more human females mated with Neanderthal males than the other way around, over thousands of years you would expect to see just what they found: more human DNA in Neanderthal X chromosomes and less Neanderthal DNA in human X chromosomes.
,
uBlock 
,
and
Ghostery 
)
