It's been 40,000 years since the Neanderthals disappeared, but their lingering genetic legacy may be influencing your health.
If you are of Asian or European descent, about 2% of your genome came from your Neanderthal ancestors, scientists say. Now, new evidence suggests this inheritance affects a broad range of health disorders including skin disease, your ability to fight infection and even your risk of addiction and depression.
"Some of the associations we found made a lot of sense when we saw them, but the ones that affected neurological and psychiatric traits -- those were surprising," said Tony Capra, a computational geneticist at Vanderbilt University in Nashville who oversaw the research.
About 50,000 years ago, the anatomically modern humans who left Africa encountered Neanderthal settlements somewhere in the Middle East, scientists believe. The question of whether the two groups interbred was debated in scientific circles for decades, until 2010 when researchers found clear evidence of Neanderthal DNA sequences in people alive today.
Since then, genetic archaeologists have been trying to determine what instructions these Neanderthal genes contain code for and why they have been preserved over so many millenniums.
The new study, published Thursday in the journal Science, is based on data collected by the eMerge network, which includes the medical records and matching DNA sequences of 28,000 people in the U.S. The researchers also worked with a previously published map of all the places where genetic variants derived from Neanderthals had been found in the human genome.
Armed with these two data sets, the team analyzed each of the 28,000 individuals in the consortium and determined whether they had the signatures of Neanderthal DNA in any of the known spots on the genome. Then, they looked for patterns that would indicate whether having these Neanderthal variants meant a person was more or less likely to have been diagnosed with a specific disease.
It stands to reason that the Neanderthal versions of genes would function differently from their modern human counterparts. After all, Neanderthals had been living in northern latitudes for thousands of years before anatomically modern humans arrived, giving the Neanderthals plenty of time to adapt to the unique environment and its pathogens.
Most geneticists believe that at least some of the Neanderthal DNA variants that remain in the human genome were able to spread because they provided some advantage to our ancestors after they left Africa.
"We know when you move a population into a new environment, the bodily systems that are involved directly with that environment are most likely to change quickly," Capra said.
Indeed, the strongest signal the researchers found involved a Neanderthal variant that improves the blood's ability to clot, or coagulate. Today, too much clotting is considered a disorder because it increases risk of stroke, pulmonary embolisms and pregnancy complications, but tens of thousands of years ago, this hypercoagulation might have served our ancestors well.
"Coagulation is one of the first immune responses the body has to a wound," Capra said. A clot not only stops bleeding, it also sends messages to the immune system "to join the fight against pathogens."
He added that the ability to form a scab quickly would have been useful for keeping unfamiliar germs out of the body.
The researchers also discovered an association between Neanderthal versions of genes and keratosis, which are skin lesions that can form after too much exposure to the sun. Keratosis is caused by a dysfunction in a type of cell called a keratinocyte that protects the skin from UV radiation. However, in the low-light conditions of the north, this mistake might have allowed more light to reach the skin, enhancing the production of vitamin D, Capra said.
Some of the findings were more difficult to explain. For example, the study authors wrote that Neanderthal variants were associated with an increased risk of mood disorders, tobacco addiction and a relatively strong effect on depression.
If you are picturing hopeless Neanderthals wandering around in a cloud of cigarette smoke, don't.
"It's very hard to project backwards," said Joshua Akey, a professor of genome sciences at the University of Washington in Seattle and a co-author on the study. "It's hard to know what the consequences of having that variation 40,000 years ago might have been."
Still, the idea that neurological and psychiatric traits are influenced by Neanderthal DNA is one of the most intriguing conclusions of the study, said Rasmus Nielsen, who studies evolutionary theory and genetics at UC Berkeley.
"That is interesting because it suggests that there were more differences in those traits between humans and Neanderthals than in other traits, suggesting perhaps that we are somewhat cognitively differentiated from Neanderthals," Nielsen said in a statement.
The researchers also found that Neanderthal DNA variants had a subtle but real association with disorders including obesity, respiratory infections, and the hardening of the arteries known as coronary atherosclerosis. However, in these cases the Neanderthal variants account for
less than 1% of the overall risk.
That may not sound like a lot, but it's significant, said Dr. Gail Jarvik, head of the Division of Medical Genetics at the University of Washington and an author on the paper.
"We now know that Neanderthal variants tweak your risk of certain disorders," she said.
At the same time, she noted that it's possible we acquired some good traits from our Neanderthal relatives.
"We looked at associations with diseases, not associations with getting along with people or other traits," she said.
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