Humans and Neanderthals Branched off 600,000 years ago Due to an Incompatible Y Chromosome
Neanderthals and humans branched off about 600,000 years ago, possibly because of genetic incompatibility in the context of the Y chromosome, a team of researchers has announced.
There was some interbreeding between Homo sapiens and Neanderthals around 100,000 to 60,000 years ago, previous research has shown.
The latest research speculates that male children conceived between a Homo sapien woman and a Neanderthal man would have resulted in miscarriage, an article in New Scientist says. The research posits that the most recent common ancestor between humans and Neanderthals lived around 590,000 years ago. This time frame is compatible with other researchers’ estimates.
A model of an adult Neanderthal male head and shoulders on display in the Hall of Human Origins in the Smithsonian Museum of Natural History in Washington, D.C. (CC BY SA 2.0)
“Neanderthal genes have been found in our genomes, on X chromosomes, and have been linked to traits such as skin color, fertility and even depression and addiction,” the article states. “Now, an analysis of a Y chromosome from a 49,000-year-old male Neanderthal found in El Sidrón, Spain, suggests the chromosome has gone extinct seemingly without leaving any trace in modern humans. This could simply be because it drifted out of the human gene pool or, as the new study suggests, it could be because genetic differences meant that hybrid offspring who had this chromosome were infertile – a genetic dead end.”
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The study offered that the woman’s immune system may have attacked the fetuses that had the Neanderthal Y chromosome due to incompatibility. Over time, the consistent miscarriages could have led to the absence of these genes in modern humans.
Stanford University’s Fernando Mendez led the research, published in the American Journal of Human Genetics, comparing ancient and modern humans, Neanderthal, and chimpanzee Y chromosomes. New Scientist says they determined mutations in four genes that could have prevented the Y chromosome to pass from Neanderthal fathers to the hybrid children.
“Some of these mutations could have played a role in the loss of Neanderthal Y chromosomes in human populations,” Mendez said.
Relationship of Neanderthal Y Chromosome to Those of Modern Humans. The genealogy (red tree) can be parsimoniously explained as mirroring the population divergence (gray tree). The researchers find no evidence for (a) a highly divergent super-archaic origin of the Neanderthal Y chromosome, (b) ancient gene flow post-dating the population split, or (c) relatively recent introgression of a modern human Y chromosome into the Neanderthal population. (Mendez et al.)
“Over the past half-decade, ancient DNA research has revealed some surprising aspects to our evolutionary history during the past 50,000 years. Perhaps the most startling of these has been the extent to which the ancestors of living people across the planet interbred with other closely related species of human. […] One particularly interesting example compared the genome of a female Neanderthal with 1,000 contemporary human ones from across the world and found clear evidence for negative selection. Mapping the DNA of Neanderthals against this large number of human genomes also showed that there were vast ‘deserts’ of Neanderthal ancestry. One million base pairs compared across the autosomes (i.e. other than the X or Y chromosomes) showed four windows in Europeans and 14 in East Asians where around 0.1% of the DNA was Neanderthal. The human Y chromosome is also known to be lacking Neanderthal DNA suggesting strong natural selection against hybrid males, who were likely to have been infertile.”
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In January 2016, a paper published in The American Journal of Human Genetics said other studies have found that modern human immunity was boosted by interbreeding with what the authors call “archaic humans.” The genes the authors studied, human toll-like receptors, were possibly passed down to modern humans when they and Neanderthals interbred around 50,000 years ago.
Comparison of Modern Human and Neanderthal skulls from the Cleveland Museum of Natural History. (Deriv) (CC BY SA 2.0)
Researchers have estimated that 1 to 6 percent of modern Eurasian genes came from now extinct hominins, including the Denosivans and Neanderthals.
Featured Image: An artist’s depiction of a family of Neanderthals. Source: Public Domain
By Mark Miller