A new study released by Howard Hughes Medical Institute at Harvard Medical School on January 29th titled “When Populations Collide” provides some interesting insights about Neanderthal DNA in modern humans. This study compared the full Neanderthal genome to that of 1004 living individuals.
In general, people in East Asia carry more Neanderthal than Europeans who carry 1-3%, and Africans carry none or very little. It appears, according to David Reich, that Neanderthal DNA is not proportionately represented in contemporary humans, meaning that some areas of Neanderthal DNA are commonly found and others not at all. Some Neanderthal genes are carried by more than 60% of Europeans or Asians, most often associated with skin and hair color, or keratin. Reich’s thought is that people exiting Africa assimilated with Neanderthals and selected for these genes that gave them an adaptive and survival advantage in the cooler non-African climate.
One particularly big Neanderthal genetic desert is the X chromosome, a phenomenon called hybrid sterility. Reich suggests that this means that when Neanderthals and humans exiting from Africa interbred, they were on the cusp of being unable to reproduce successfully. Reich explains that “when two populations are distantly related, genes related to fertility inherited on the X chromosome can interact poorly with genes elsewhere in the genome and that interference can render males, who carry only one X, sterile.”
Given the recent discussions about the X chromosome and the possibility that it may be inherited in an all-or-nothing manner more often than the other chromosomes, I had to wonder how they determined that this was hybrid sterility and not an case of absence of recombination.
Reich’s team apparently had the same question, so they evaluated the genes related to the function of the testes, confirming they too had a particularly low inheritance frequency of Neanderthal DNA. These, combined, would eventually cause the X to be present in very small quantities in the genome of descendants since the Neanderthal X could only be inherited from women and then would cause the resulting males to be sterile. So in essence, only females could pass the X on and only their daughters would pass it further. Males carrying that X not only wouldn’t pass the X, they wouldn’t pass anything at all due to sterility.
If, in addition to this, the X has unusual recombination features, that could exacerbate the situation. Conversely, if the X is inherited intact more often than not at all, it could increase the likelihood of the X being brought forward in the population.
Reich says his team is now focused on looking at Neanderthal DNA and human disease genes. He says that his new study revealed that lupus, diabetes and Crohn’s Disease likely originate from Neanderthals.
Another study, published the same day in Science titled “Resurrecting Surviving Neandertal Lineages from Modern Human Genomes,” reaches the same conclusions about the Neanderthal inherited traits related to skin color. This study compared the full genomes of 379 East Asians and 286 Europeans to Neanderthal genomes and discovered that they could map about 20% of the Neanderthal DNA in those individuals today. This, conversely, means that 80% of the Neanderthal genome is missing, so either truly missing or simply missing in the people whose DNA they sequenced. It will be interesting to see what is found as more contemporary genetic sequences are compared against Neanderthal, and as more Neanderthal DNA is found and sequenced.
Fortunately, recent advances in dealing with contaminated ancient DNA hold a great deal of promise in terms of increasing our ability to sequence DNA that was previously thought to be useless. This report is described in the article “Separating endogenous ancient DNA from modern day contamination in a Siberian Neanderthal” and was used in the sequencing and analysis of the Neanderthal toe bone found in Siberia.
To better understand the legacy of Neanderthals, Dr. Reich and his colleagues are collaborating with the UK Biobank, which collects genetic information from hundreds of thousands of volunteers. The scientists will search for Neanderthal genetic markers, and investigate whether Neanderthal genes cause any noticeable differences in anything from weight to blood pressure to scores on memory tests.
“This experiment of nature has been done,” says Dr. Reich, “and we can study it.”