A new study says a person's risk of becoming depressed or hooked on smoking may be influenced by DNA inherited from Neanderthals.
Researchers found evidence that one bit of Neanderthal DNA can boost the risk of tobacco addiction, while others can slightly raise or lower the risk of being diagnosed with depression.
It's the latest in a series of studies of the Neanderthal genetic heritage in modern people. Past studies have suggested it raises risk of allergies, for example.
Neanderthals and modern people split off from each other on the evolutionary tree hundreds of thousands of years ago. But ancestors of modern people interbred with Neanderthals about 50,000 years ago after leaving Africa. So in people of Asian or European ancestry, around 2 percent of DNA can be traced to Neanderthals.
Studying that DNA might help give insights into the biological roots of some diseases, said Tony Capra, an evolutionary geneticist at Vanderbilt University. He is senior author of the study, which was released Thursday by the journal Science.
The DNA linked to depression or tobacco addiction affects risk, and doesn't by itself produce those conditions.
The researchers focused on bits of Neanderthal DNA that had been identified in prior research. They looked for effects from about 1,500 of them in medical records of some 28,000 Americans of European ancestry, for whom they had genetic information.
One analysis supported a previously suggested influence on the risk of depression, with some DNA bits raising the risk and others lowering it. Overall, the analysis found, this DNA affected the risk by about 1 percent. For an individual, the effect would depend on just which bits that person carried, Capra said.
A second analysis found evidence that a particular bit of Neanderthal DNA, one quite rare in the population studied, roughly doubled the risk of getting hooked on smoking.
Tobacco was not available to Neanderthals, so "they were not walking around puffing on cigarettes," Capra told a press conference Thursday. It's hard to say what effect, if any, that bit of DNA had on them, he said in a telephone interview.
Kenneth Kendler, a genetics expert at Virginia Commonwealth University who didn't participate in the study, said he was skeptical that the effect on smoking addiction could be so strong, given previous studies of genetic influence on tobacco behavior.
Overall, Kendler said he found the evidence suggestive but not convincing for the proposed influences on depression and tobacco use.
The study also linked Neanderthal DNA to risk for some other modern-day conditions, such as scaly skin patches called actinic keratoses, urinary system symptoms and an unusually strong tendency toward blood clotting.
Sriram Sankararaman of the University of California, Los Angeles, who led a 2014 study of Neanderthal DNA in modern people, said he found the new work "really exciting."
Capra said the Neanderthal DNA picked up by early humans may have helped them adapt and survive in their ancient environment. But in today's world, he said, most of that genetic legacy is either neutral or mildly harmful.
So for people of Eurasian ancestry, he said, "at least today, Neanderthal interbreeding was not so good to us."