That the forebears of modern Homo sapiens and the long-extinct Neanderthals lived side by side is well known: that they interbred, and that up to 4% of modern human DNA is inherited from the first Europeans, was confirmed only in 2010.
US researchers examined a database of 28,000 patients whose biological samples had been linked to versions of their medical records. Identities remained anonymous but the researchers could see how inheritance linked to medical history.
Then, they report in the journal Science, they matched the modern human database against a map of those groups of genes known to have been inherited from the big-boned, heavy-browed, red-haired humans whose ancestors had moved out of Africa long before Homo sapiens, and colonised Ice Age Europe.
“Our main finding is that Neanderthal DNA does influence clinical traits in modern humans,” said John Capra, an evolutionary geneticist at Vanderbilt University in Nashville, Tennessee. “We discovered associations between Neanderthal DNA and a wide range of traits, including immunological, dermatological, neurological, psychiatric and reproductive diseases.”
Sub-Saharan African peoples do not inherit Neanderthal DNA. The assumption is that the Neanderthals left Africa first, had time to adapt to a colder, darker and more difficult world, evolved paler skin colour to take advantage of less certain sunlight, and developed other traits that might have helped them survive changing conditions.
Early modern humans – more gracile, and perhaps quicker to adapt and take advantage of their environment – then migrated north from Africa to outpace and outlive the first Europeans. But, during the thousands of years the two species coexisted, they also interbred.
And these encounters passed on traits that might have been of some evolutionary advantage in an Ice Age world. But in changing conditions, the same lengths of inherited DNA contained greater health liabilities as well.
One of these, the researchers think, was a Neanderthal gene variant that increases blood-clotting. This would have sealed wounds more quickly, and prevented infection more easily. But in a modern western society, hyper-coagulation brings other problems, including greater risk for stroke, pulmonary embolism and pregnancy complications.
One length of Neanderthal DNA is now linked to increased risk of nicotine addiction, and several variants influence the risk of mood disorders, including depression. As tobacco was introduced into widespread use in Europe only 400 years ago, the researchers were surprised at the number of Neanderthal genetic variants now associated with modern psychiatric and neurological disorders.
“The brain is incredibly complex, so it is reasonable to expect that introducing changes from a different evolutionary path might have negative consequences,” said Corinne Simonti, a Vanderbilt doctoral student and the study’s first author.