All Europeans and Asians, however, retain Neanderthal genes affecting keratin, which is a fibrous protein in skin and hair. Reich explained that keratin makes skin, hair and nails tougher and better able to withstand cold temperatures.
“It’s tempting to think that Neanderthals were already adapted to the non-African environment and provided this genetic benefit to humans,” Reich said.
It's possible that the lighter skin and hair of many Europeans was also influenced by Neanderthal ancestry, but Sankararaman indicated that further research is needed to fully make that connection.
The study additionally found that genetic variants passed down from Neanderthals also affect an individual’s disposition toward type 2 diabetes, Crohn’s disease, lupus, a liver condition known as biliary cirrhosis and even whether a person is likely to smoke.
X chromosome-related genetic mutations from Neanderthals, along with DNA associated with male sexual organs, were more likely to have been incompatible with the human genome. These were less easily exchanged between the two species. People who still retain such Neanderthal DNA, like East Asians, experience reduced fertility as a probable result.
“This suggests that when ancient humans met and mixed with Neanderthals, the two species were at the edge of biological incompatibility,” Reich said.
Daven Presgraves, an associate professor in the University of Rochester’s Department of Biology said the Nature paper implies humans migrating out of Africa were able to “borrow” genes associated with skin, hair and nails from Neanderthals “perhaps accelerating adaptation to a Eurasian environment that was new to them.”
The findings also strongly support the theory that Neanderthals were simply absorbed into the human gene pool. They are now, in essence, a part of many people.