When we interbred with Neanderthals, it was usually the fathers

When our species and Neanderthals interbred, it may have been mostly female Homo sapiens and Neanderthal males who mated. That’s the conclusion of a study of the genetic traces left by interbreeding in both populations.

It is unclear why this sex-biased mating pattern would occur. It is possible that Neanderthal men preferred women H. sapiens over females of own species or over this female H. sapiens females preferred Neanderthal males or both. There is also no way to determine whether the mating was consensual or forced.

“I think there’s very little we can say,” he says Alexander Platt at the University of Pennsylvania in Philadelphia. “The meaningful thing we can say is that it was something that happened for generations.”

Other geneticists say the evidence is interesting but inconclusive. “I think we need more evidence because it’s a big behavioral claim,” he says Arev Sumer at the Max Planck Institute for Evolutionary Anthropology in Leipzig, Germany.

We’ve known that since 2010 H. sapiens — sometimes referred to as modern humans — and Neanderthals interbred after some members of our species moved from Africa to Eurasia. There appear to have been several periods of interbreeding: one about 50,000 to 43,000 years ago, and an earlier one that is harder to determine, perhaps 200,000 years or more in the past. Today, all people of non-African descent carry some Neanderthal DNA.

However, scientists did not pay much attention to how this affected the sex chromosomes. Females typically have two X chromosomes, while males typically have one X and one Y. Platt and colleagues Sarah Tishkoff and Daniel Harrisalso at the University of Pennsylvania, focusing on human and Neanderthal X chromosomes.

“One of the things that was very clear when looking at the human X chromosome is that it’s almost entirely Neanderthal desert,” says Harris. Compared to other chromosomes, the human X chromosome is virtually devoid of Neanderthal DNA. The team considered four possible reasons.

First, it could be that H. sapiens and Neanderthals were so different that their DNA didn’t work very well together. This is called hybrid incompatibility and can cause human-Neanderthal hybrids to have health or reproductive problems. However, the team found that the Neanderthal X chromosome contained more H. sapiens DNA than that of Neanderthal non-sex chromosomes—suggesting that the X-chromosome DNA from the two hominins was compatible.

Second, natural selection may have favored modern human DNA. Neanderthals had a relatively small population, so natural selection would try to remove harmful mutations. In contrast, modern humans had larger populations and the bad mutations would have been removed, so the DNA of the modern human X chromosome would have spread throughout the Neanderthal population. But the team says that doesn’t make sense either, because the modern human DNA retained on the Neanderthal X chromosome is mostly in non-functional areas where it wouldn’t do any good.

Alternatively, it could be a cultural reason. Societies differ in how the sexes move: in some, women leave their home group to live with their sexual partners, but in others, men relocate. Modern human females moving in to live with Neanderthals could cause a bias on the X chromosome, but that wouldn’t be enough to explain the strength of the bias the team found—even if every single interbreeding female was a modern human.

That, the team says, leaves only one possible explanation: mating preference. Neanderthal males preferred females H. sapiens over Neanderthal partners, or females H. sapiens they preferred Neanderthal men to human partners—or both. “If they like it that much, that explains everything,” says Platt.

However, other geneticists argue that we cannot be so sure that alternative explanations are wrong. Sümer points out that an earlier episode of interbreeding had a drastic effect on the Neanderthal genome: their original Y chromosomes were completely replaced according to H. sapiens Y chromosomes. “A huge number of modern human males should be involved in this mixing process,” he says.

We can’t rule out hybrid incompatibility either, he says Moisès Coll Macià at the Institute of Evolutionary Biology in Barcelona, ​​Spain. The team hypothesized that it would be just as intense when Neanderthal DNA entered the modern human genome and vice versa. “That may not be the case,” he says.

Coll Macià says we need to consider yet another possibility: meiotic drive. Rogue genetic elements can cause one of a pair of chromosomes to be passed on more often than would be expected by chance. His team found him preliminary evidence This happens in modern humans outside of Africa, resulting in the removal of Neanderthal DNA from their X chromosomes.

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