This is in contrast to genes that are transferred on other chromosomes and mitochondrial genes. Hypothesis: Homo sapiens women aborted fetuses with Neanderthal characteristics. Perhaps it is related to the fact that these genes play a role in the immune system in the part responsible for rejecting or accepting transplanted organs
Although it has long been known that modern humans carry traces of Neanderthal DNA, a new international study led by researchers from the Stanford University School of Medicine has revealed that Neanderthal-specific genes found on the Y chromosome disappeared from the human genome long ago.
The study was published on April 7 in The American Journal of Human Genetics. The senior author is Carlos Bustamante, a professor of bioinformatics and genetics at the Stanford School of Medicine, and the lead author is Fernando Mendes, a Bustamante postdoctoral fellow.
The Y chromosome is one of the two sex chromosomes in humans. Unlike the X chromosome, the Y chromosome is passed down from the white father only. This is the first study to look at the Neanderthal Y chromosome, Mendez said. Previous studies have looked at DNA sequences from Neanderthal female fossils or from mitochondrial DNA, which is passed on to children from their mothers.
Other studies have shown that modern human DNA contains 2.5-4% Neanderthal DNA, the result of interbreeding between modern humans and Neanderthals 50 years ago. The team members were surprised to discover that, unlike other types of DNA, Y chromosomes appear to have not been transferred to modern humans at that time.
"Genes from the Neanderthal Y chromosome have never been observed in any human sample ever tested," Bustamante said. "This does not prove that these genes are completely extinct, but it is likely."
Why not Neanderthal DNA?
The reason for the disappearance of the genes is unknown. The Neanderthal Y-chromosome genes may have simply slowly disappeared from the human genome pool at random over thousands of years. Another possibility, Mendez says, is that the Neanderthal Y chromosomes contained genes that do not correspond to the rest of the human genes, and that he and his colleagues have discovered evidence that supports this hypothesis. Indeed, one of these Y-chromosome genes that differed in Neanderthals was found to have an effect on transplant rejection when men donate organs to women.
"The functional nature of the mutations we found," said Bustamante, "may suggest that Neanderthal Y chromosome sequences may have played a role in blocking gene transfer, but we need to conduct more experiments to prove this."
However, a number of genes were discovered on the Neanderthal Y chromosome that do not differ from those in modern humans - genes that function as part of the immune system. The three genes are HY antibodies similar to the HLA antibodies that surgeons test to ensure that the organ donor and transplant have similar immune profiles. Because these Neanderthal antigens are on the Y chromosome they are male specific.
Theoretically, Mendez says, a woman's immune system could attack a male fetus carrying Neanderthal HY genes. If the women consistently aborted male embryos carrying Neanderthal Y chromosomes, this could explain their absence in modern humans. So far this is only a hypothesis, but it is known that the immune system of modern women sometimes reacts to male fetuses in cases of genetic incompatibility.
when did we break up
Y chromosome data may shed new light on the timeline of when modern humans and Neanderthals split. The human lineage split from other apes over several million years, ending no later than about 4 million years ago. After the final split from other apes, the human lineage branched into a series of different types of humans, including separate lineages that led to Neanderthals on the one hand and modern humans on the other.
Previous estimates based on mitochondrial DNA placed the split point between 400 and 800 thousand years ago. The current study, based on the DNA of the Y chromosome, places the splitting point about 550 thousand years ago.
Analysis of the Neanderthal Y chromosome sequence may shed additional light on the relationship between humans and Neanderthals. Another challenge for the research team is to find out whether the human Y chromosome still has genes that survived from the Neanderthals.
The data for the study came from public garden databases.