Neanderthals were wiped out about 40 000 years ago, but some of their genes live on in modern humans. And scientists are learning more about what that might mean for our health.
Researchers have known for some time that many people carry bits of Neanderthal DNA.
In fact, if you're of European or Asian ancestry, your genes are likely between 1% and 2% Neanderthal, said Rajiv McCoy, the first author of the new study.
And while most people have heard of Neanderthals, they often have mistaken beliefs about them, noted McCoy, a postdoctoral researcher at the University of Washington in Seattle.
The study was published in the journal Cell.
Neanderthals are not our ancestors
"A lot of people think Neanderthals were an ancestor of modern humans," he said.
In reality, they walked the Earth at the same time as our modern human ancestors – and roughly 50 000 years ago, started mating with them, historians say. That happened after modern humans started migrating from Africa to Europe and Asia, which is why Neanderthal DNA is mainly found in non-Africans today.
The big question is, what are those Neanderthal genes doing? So far, studies have found associations between some of the genes and increased risk of depression, lupus, blood clots, allergies, certain skin lesions and even nicotine addiction, the researchers pointed out.
DNA testing has many advantages, as in the identification of health risks. One example is the fact that approximately 10% of the population metabolise codeine at a higher rate than everyone else – something that can be picked up by genetic tests.
Neanderthal DNA affects human genes
But those are just correlations. Now McCoy and his colleagues have found some hints as to how Neanderthal DNA might influence human traits and health: It may affect how genes are turned "on" or "off".
"This begins to help us connect the dots," McCoy said. "What's the mechanism behind these correlations?"
That understanding could eventually help researchers get a better handle on how certain human traits and diseases arise, said McCoy.
For the study, the researchers analysed RNA from a large database of human tissue samples. RNA is a molecule that carries the instructions from DNA to the machinery in our body cells that makes proteins.
McCoy and his colleagues looked for people who carried both a Neanderthal variant and a modern human variant of any given gene. (We all carry two copies of each of our genes, one inherited from each parent.)
Then, for each gene, the researchers looked at whether the Neanderthal and human versions differed in their "expression" – or level of activity – in 52 different body tissues.
The investigators found differences in the genes' expression in about one-quarter of the tissue types they tested.
One example the researchers point to is a gene called ADAMTSL3. The Neanderthal variant has been linked to a decreased risk of schizophrenia, and also correlates with height.
This study found that the Neanderthal version is less-expressed. When that happens, cells make a somewhat modified protein. It's not clear, however, how that modified protein is connected to schizophrenia or height, the researchers said.
The study also found that, overall, Neanderthal genes are least active in the brain and testes.
It's not clear why, McCoy said. But one theory is that genes in those tissues have evolved at a more rapid pace.
Whatever the reason, it appears "there's a pressure to not have Neanderthal DNA around" in the brain and testes, said Tony Capra, an assistant professor of biological sciences at Vanderbilt University in Nashville.
Capra has also studied the remnants of DNA left behind by the Neanderthals.
Why did Neanderthals not survive?
"One reason," he said, "is fundamental curiosity about this group that had so much in common with us, but did not survive."
Plus, Capra said, any knowledge gained from this research could be "medically relevant in some cases".
Capra said the new findings shed light on how Neanderthal gene variants may be related to diseases.
Of course, McCoy pointed out, complex conditions like depression, schizophrenia and addiction are influenced by many factors. "The contribution of a Neanderthal gene would be small," he noted.
And, Capra said, Neanderthal genes are not all bad, either. It's likely the DNA had some upsides for prehistoric humans.
For example, having blood that easily clots may be a bad thing in modern times, when so many people suffer heart attacks and strokes. But for prehistoric people, who would often die of wounds, fast-clotting blood could have been a major advantage.
Similarly, the effects of Neanderthal genes on the immune system and skin were probably a boon back in the day, Capra said. Today, the trade-off may be increased risks of allergies and autoimmune diseases.
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