Researchers have long known that Neanderthal DNA resides within us. However, scientists have long debated how much DNA can be found. Now, researchers have discovered that the Neanderthal Y-chromosome genes actually disappeared from the human genome long ago.
Previous research has shown that the DNA of modern humans is from 2.5 to four percent Neanderthal DNA. This is largely due to modern humans breeding with Neanderthals about 50,000 years ago. In this case, though, the researchers wanted to see exactly what DNA was passed along.
The Y chromosome is one of two human sex chromosomes. This chromosome is passed exclusively from father to son. This means that understanding the Neanderthal Y chromosome may allow researchers to better understand when Neanderthals interbred with humans.
In this case, the researchers found that the Neanderthal Y chromosome DNA was not passed to modern humans during the time that Neanderthals and humans interbred.
But why was this? It could be that the Neanderthal Y chromosome genes simply drifted out of the human gene pool by chance over the millennia. It's also possible that Neanderthal Y chromosomes include genes that aren't compatible with other human genes. In fact, scientists have actually found that Y chromosome genes that differ in Neanderthals were implicated in transplant rejection when males donate organs to women.
"The functional nature of the mutations we found suggests to us that Neanderthal Y chromosome sequences may have played a role in barriers to gene flow, but we need to do experiments to demonstrate this and are working to plan these now," said Carlos Bustamante, one of the researchers of the new study.
Several Neanderthal Y chromosome genes that differ from those in humans are actually part of the immune system. Three are "minor histocompatibility antigens," or H-Y genes. In theory, it's possible that a human woman's immune system may attack a male fetus carrying Neanderthal H-Y genes. This, in turn, would likely cause a woman to miscarry male babies carrying Neanderthal Y chromosomes, which would explain its absence today.
The findings reveal a bit more about humans' history with Neanderthals. This is important not only for understanding Neanderthal history, but also for understanding the genetic history of humans.
The findings were published in the April 2016 issue of the American Journal of Human Genetics.
