Humans and Neanderthals interbred — but it was mostly male Neanderthals and female humans who coupled up, study finds
When Neanderthals and modern humans first came together, they preferred pairings between Neanderthals and human women, a new study of ancient and modern genomes suggests. This discovery helps explain why modern humans (Homo sapiens) have a relatively low level of Neanderthal genes and why these genes are found today in some populations and not others.
Since the first genomes of modern humans and Neanderthals were sequenced more than 20 years ago, scientists have wondered about “Neanderthal deserts,” or places in the modern human genome where Neanderthal genes are rare. The two groups crossed paths for a few periods after their ancestors separated 600,000 years ago. The result is that most non-African peoples on the planet today wear a on average 2% Neanderthal DNAwhile some African groups have up to 1.5%inherited from H. sapiens who mixed with Neanderthals in Eurasia and then moved to Africa.
But what has baffled experts is that the genes inherited from Neanderthals are found only in tiny areas of our X chromosome, even though these genes appear in greater numbers on our other chromosomes. There are regions on the X chromosome – the sex chromosome of which every human has at least one copy – where no living human has Neanderthal ancestry.
“For years we simply assumed that these deserts existed because certain Neanderthal genes were biologically ‘toxic’ to humans – as tends to be the case when species diverge – so we thought that these genes may have caused health problems and had probably been purged by natural selection.” Alexander Platta population geneticist at the University of Pennsylvania, said in a statement.
But in a study published Thursday February 26 in the journal SciencePlatt and his colleagues concluded that the most plausible explanation for these “Neanderthal deserts” was actually mate preference, an evolutionary mechanism that plays a major role in sexual selection. Biologists commonly illustrate the evolutionary result of mate preference with the large, colorful tail of the male peacock. Early humans and Neanderthals probably also chose their partners for specific reasons.
In-depth DNA analysis
The researchers analyzed the genomes of 73 women from three modern African populations without Neanderthal ancestry, including the !Xoo, Ju|’hoansi and Khoisanand compared them to the genomes of some Neanderthals. First, they looked at Neanderthal X chromosomes and found significantly higher amounts of modern human ancestry there than on other Neanderthal chromosomes. This result revealed that the absence of Neanderthal genes in the human X chromosome is not the result of incompatibility, which would have suggested that Neanderthal genes caused problems for modern humans and were eliminated by natural selection.
The surprisingly high amount of modern human DNA fragments found in Neanderthals may instead be explained by mate preference, the researchers concluded. Since females carry two X chromosomes and males only carry one, there is a mating preference between females. H. sapiens and Neanderthals would mean that fewer Neanderthal X chromosomes would enter the human gene pool, producing the pattern the researchers identified in the genomes.
But the reasons for this partner preference – and its orientation – remain elusive.
“I have no idea whose preference is being expressed here,” Platt told Live Science in an email.
Previous research in the Neanderthal Y chromosome — one of the two sex chromosomes of male individuals — indicates that there was interbreeding between male individuals. H. sapiens and Neanderthal women. But the new study shows that in reality, Neanderthals and women H. sapiens loved each other more than Neanderthal women and men H. sapiens did.
“We just don’t have a genetic signature to discern beyond that at the moment,” Platt said.
The researchers did not rule out more complex evolutionary scenarios that could have combined natural selection, sexual bias, mate preference and sex-specific migration to contribute to the “Neanderthal deserts” of the human genome.
Answering questions about the structure of modern Neanderthal and human societies is also important for a better understanding of mate choice in the past, because anthropologists and evolutionary biologists who have studied the phenomenon show that partner choice is partially learned.
The research team plans to “examine the evolution of social structures and gender roles within Neanderthals,” which “could possibly shed light on the situation,” Platt said. “But I think we are far from knowing.”
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