Neanderthals' human blood mostly came from moms, study suggests

patch it's mutual knowledge that Neanderthals and humans interbred long agone, a young genetical study is suggesting that among the Neanderthals, their human blood came mostly from the moms in their family.

That was likely due to differences in what Neanderthal men and women preferred in a mate.

A team of researchers from the University of Pennsylvania came to that conclusion by looking at the amount of human and Neanderthal DNA in the X chromosomes of Neanderthals. The study published Thursday in the journal Science tells us something beyond Neanderthal biology and genetics, the researchers suggest.

"It allows us to say something about the social behaviour of these ancient populations," said Sarah Tishkoff, professor of genetics and biology and head of the lab that did the analysis. 

Modern humans originated in what's now Africa, while Neanderthals lived in Europe and Asia. They encountered each other and interbred a number of times, leaving each other's genes in their genomes. That's why all humans with ancestors outside sub-Saharan Africa have some Neanderthal DNA.

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Similarly, Neanderthal genomes after a period of interbreeding with humans 250,000 years ago have some human DNA. 

But it's not evenly spread out through their genes and chromosomes.

In this particular study, researchers took a closer look at the X chromosome, which is unique because men only have one — all other chromosomes come in pairs in both men and women. The researchers compared the X chromosomes in three ancient Neanderthal women (who each have two X chromosomes) and 73 modern human women from sub-Saharan Africa (who have no Neanderthal genes).

What they found was that the Neanderthal women had far more human DNA in their X chromosomes than their other chromosomes (called autosomes) — up to 62 per cent more in the case of the oldest Neanderthal genome, belonging to a woman who lived 122,000 years ago. 

Alexander Platt, a senior research scientist and evolutionary biologist in Tishkoff's lab, came up with some possible explanations. 

One option was that human genes on the X chromosomes worked better in Neanderthals than Neanderthal genes did, and replaced theirs. If that was the case, there would be more human DNA in regions of the chromosome with important genes.

"That's not what we found," Platt said. In fact, there was more human DNA in parts of the genome "that aren't really doing anything."

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Another possible explanation is that mating between men with more Neanderthal ancestry and women with more human ancestry was more common than vice versa.

That would result in more human DNA in the X chromosome compared to other chromosomes,  because women provide twice as many X chromosomes to the next generation as men do. Women pass X chromosomes to all their children, whereas men only pass their X chromosome on to their daughters.

The team found that this kind of unequal pairing — and unequal passing-on of the X chromosome — could explain what they found. 

They then tried to come up with ways that kind of unequal pairing could happen.

"One very simple one ... Is if there is a preference such that anatomically modern human ancestry is simply more appealing when it is found in females than when it's found in males," Platt said.

"Or the other way around, Neanderthal ancestry makes for a more appealing person when it's found in males than when it's found in females."

Different migration patterns for men and women alone didn't seem provide a complete explanation, as the unequal pairing would have to happen over many generations.

The researchers said there may actually have been a combination of factors that led to the biases we see in Neanderthal X chromosomes — including mate preferences, different migration patterns among men and women and natural selection through certain kinds of genes.

Damian Labuda, a retired professor at the University of Montreal, has used in his own research some of the human genetic datasets that Tishkoff collected. He had previously found the presence of Neanderthal DNA on the human X chromosome.

Labuda called the new study "fascinating."

He said the explanation makes sense and doesn't exclude the possibility that other factors could influence breeding patterns, or that multiple factors could work together. 

He pointed to recent research suggesting that Neanderthal-human hybrid mothers were less likely to have healthy babies, thanks to a mismatch between the ability of humans and Neanderthals to carry oxygen in their red blood cells. 

Another, earlier study had suggested that Neanderthal-human hybrid men weren't very fertile, based on the fact that in humans, there are no Neanderthal genes in the part of the X chromosome associated with the testicles.

Labuda said ancient Neanderthals may have realized that certain kinds of pairings resulted in fewer healthy babies and that may in turn have shaped their mating preferences or even taboos.

"These people were very good observers. Their intellect was like ours, so they would have noticed that things like this happen."

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