While it is understood that modern humans still carry traces of Neanderthal DNA, scientists now suggest that Neanderthal Y-chromosome genes have long disappeared from the human genome.
An international team, led by researchers at the Stanford University School of Medicine, say that interbreeding was not as seamless as it appears, with some genetic incompatibility between the two species that would of led to miscarriages of male offspring.
The researchers have identified four genes that may have prohibited the male sex chromosome from being passed to human/Neanderthal hybrid babies. Three of the genes resemble those that are known to cause a mother to have an immune reaction against a male fetus, causing a miscarriage in modern humans.
The Y chromosome, unlike the X chromosome, is passed solely from father to son. Lead author of the study Fernando Mendez, PhD, and a postdoctoral scholar at Stanford, said that this was the first study to examine a Neanderthal Y chromosome.
Previous research has only sequenced DNA from the fossils of Neanderthal women or from mitochondrial DNA, which is passed to children of either sex from their mother, according to Stanford Medicine.
Other studies have revealed that 2.5 to 4 percent of modern human DNA is that of Neanderthal DNA. This is the inheritance of the inter-breeding between modern humans and Neanderthals 50,000 years ago.
The researchers were enthusiastic to find out that unlike other types of DNA the Neanderthal Y chromosome DNA was seemingly not passed on to modern humans during this time. Senior author Carlos Bustamante, PhD, professor of biomedical data science and of genetics at the School of Medicine, said in a statement;
“We’ve never observed the Neanderthal Y chromosome DNA in any human sample ever tested. That doesn’t prove it’s totally extinct, but it likely is.”
They examined genes on the Y chromosome of a 49,000-year-old Neanderthal male found in El Sidron, Spain.
The researchers remain unclear why there is no Neanderthal DNA, however they do have theories. One reason could be that the Neanderthal Y chromosome genes may have purely wandered out of the human gene pool by chance.
Another possibility is that the Neanderthal Y chromosomes could have included genes that are unsuited to other human genes; the researchers have found evidence supporting this theory. According to Stanford Medicine, one of the Y chromosome genes that differ in Neanderthals has previously been implicated in transplant rejection when males donate organs to women.
The new study also supports earlier data based on mitochondrial DNA, which put the divergence of the human and Neanderthal lineages at between 400,000 and 800,000 years ago. The last common ancestor of Neanderthals and humans according to the study was about 550,000 years ago, with the researchers believing that Neanderthals had died out about 40,000 years ago.
By sequencing the Neanderthal Y chromosome, it is hoped that a better understanding will be gained on the relationship between humans and Neanderthals. The study titled “The Divergence of Neandertal and Modern Human Y Chromosomes” was published in The American Journal of Human Genetics.