The amorous unions between modern humans and Neanderthals may have led to sons who weren't much good at fathering children themselves, a new study suggests. The findings hint that hybrid boys were partially infertile or perhaps entirely sterile due to the incompatibility of human and Neanderthal DNA. Bolstering those results, a second new study finds that some of the Neanderthal DNA that entered the human genome as a result of interbreeding seems to have made for more feeble offspring.

But both studies also find evidence that Neanderthals bequeathed useful DNA to humans ?? DNA that seems to have helped Homo sapiens adapt to new locales after they left their homeland in Africa. Whether the interbreeding was a net gain or a net loss for humans may never be determined, say the scientists involved.

"It's impossible to come to a simple conclusion like 'It was beneficial' or 'It was deleterious,' or 'It was not helpful,' " says University of Washington evolutionary geneticist Joshua Akey, an author of one of the new papers. "It was all of those things simultaneously. In different parts of our genome, (mixing) was advantageous. In other parts of our genome, it was not a good thing."

When modern humans moved out of Africa into Eurasia some 100,000 years ago, they found Neanderthals there to greet them. The two groups may have made war, but they certainly also made love. Today's Europeans and East Asians owe 1% to 2% of their DNA to Neanderthals, but the impact of those additions has been unclear.

To find out more, rival teams used different methods to conduct the first systematic surveys for Neanderthal genetic material in the DNA of modern humans. Despite their different techniques, both teams found evidence of Neanderthal DNA in genome regions involved with the production of keratin, a protein in skin and hair - a sign that the Neanderthal DNA was likely to have been beneficial. Perhaps the Neanderthal DNA helped make skin and hair more suitable for the Eurasian climate, or more resistant to the local germs. One set of findings was reported in this week's Nature, the other by Akey and a colleague in this week's Science.

Before modern humans arrived in Eurasia, "Neanderthals were living (there) for hundreds of thousands of years, and so they had genetics that were adapted to the environment," says statistical geneticist Sriram Sankararaman of Harvard Medical School, an author of the Nature paper. "Modern humans were moving into these same areas, and the genes they acquired from Neanderthals could have been beneficial." His group also found Neanderthal DNA in areas of the human genome that affect diseases such as type-2 diabetes, but the researchers can't say exactly how the Neanderthal genetic material affects human health today.

Both teams also found evidence that human-Neanderthal mating wasn't always good for the resulting children. Long stretches of DNA in living humans are devoid of Neanderthal DNA, suggesting it was purged from the human genome because of its negative effects. Perhaps offspring with the Neanderthal DNA were less likely to survive adulthood, or perhaps they were less likely to have children of their own. The Nature study indicates that some Neanderthal DNA, when introduced to the modern-human genome, led to male children with lower fertility.

That's a surprising result, says population geneticist Montgomery Slatkin of the University of California, Berkeley, who was not involved with the new research.

"I honestly thought (Neanderthals and modern humans) could interbreed freely, in the same way that different groups of modern humans can interbreed freely," Slatkin says. "And that is evidently not the case."

Instead the results "seem to confirm that Neanderthals and moderns were basically on separate evolutionary trajectories despite a little hanky-panky along the way," Ian Tattersall, curator emeritus at the American Museum of Natural History, says via e-mail.

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Neanderthal, human mixing had gene benefits, drawbacks