It’s well established that Homo sapiens and Neanderthals interbred when the two species met one another hundreds of thousands of years ago. In fact, studies over the last half decade show that non-Africans get about 2.5 to 4 percent of their DNA from Neanderthals and that their genes are important to some immune system functions. Scientists even recently figured out roughly when and where the two species got it on.
But that interbreeding was probably not as seamless as it appears, according to a new study in The American Journal of Human Genetics. There may have been some genetic incompatibility between the two species that led to miscarriages of male offspring.
Carlos Bustamante, professor of biomedical data science and genetics at Stanford and his team realized that all of the Neanderthal genes found in modern humans come from X chromosomes. They examined genes on the Y chromosome of a 49,000-year-old Neanderthal male found El Sidron, Spain, and compared them to modern human populations, but found no trace of the Neanderthal Y chromosome genes.
“We’ve never observed the Neanderthal Y chromosome DNA in any human sample ever tested,” Bustamante says in a press release. “That doesn't prove it’s totally extinct, but it likely is.”
This led the team to investigate why the Neanderthal Y could have died out in the human populations while X-linked chromosomes survived.
There's an idea that incompatibility between the genes of Neanderthals and humans did not always produce viable offspring, Fernando Mendez, lead author on the paper, explains to Eva Botkin-Kowacki at The Christian Science Monitor.
Examining the Neanderthal Y chromosome, the researchers identified four genes that could have prevented the male sex chromosome from being passed to human/Neanderthal hybrid children. Three of the genes resemble those in modern humans that can cause a mother to have an immune reaction against a male fetus, causing a miscarriage.
This may have not caused a miscarriage every time. But “even if the effect was weak, the Y-chromosome from Neanderthals would have had a harder time being passed every generation." Mendez tells Botwin-Kowacki. "It would have been removed eventually.”
“I think the observation that you don't see the Neanderthal Y-chromosome at all is consistent with the idea that there were fitness consequences to hybridization," University of Washington geneticist Joshua Akey tells the Monitor.
Even so, more research needs to be done to say for sure if the different species could successfully produce male offspring. Bustamante and Mendez hope to do more sequencing of the Neanderthal Y chromosome to look more deeply at the probability of successful breeding between ancient humans and Neanderthals.