Having Neanderthal DNA in your genes may increase the risk for depression, nicotine addiction, stroke, pregnancy complications and many other health problems, a new study suggests.

The study, published in the journal Science, shows how ancient liaisons between Neanderthals and anatomically modern humans continue to impact our genetic heritage.

Previous studies have suggested that when early Homo sapiensmigrated out of Africa they interbred with different species in Asia and Europe.

Sequencing of the Neanderthal genome has made it possible to link Neanderthal DNA to aspects of appearance and the health risks of today’s Europeans and Asians.

“This study has modern-day clinical relevance, because it reveals how evolutionary history has led to some differences in disease risk between populations,” senior author Assistant Professor John Anthony ‘Tony’ Capra said.

“In terms of treating these diseases, it will be important to understand how these bits of Neanderthal DNA exert their influence at the molecular level.”

Health records matched to genomes

Dr Capra, an evolutionary geneticist and assistant professor of biological sciences at Vanderbilt University, and his colleagues analysed a database containing the anonymous health records of 28,000 US patients.

The scientists next looked at the genomes of each person, focusing on their Neanderthal DNA, and then compared the two sets of data to see how the DNA had influenced the patients’ risks for the different health problems.

A finding is “that having Neanderthal DNA at one location in the genome significantly increased risk for blood hyper-coagulation,” Dr Capra said.

Back in the day, this trait might have helped early humans to seal wounds more quickly, preventing infections, but now it can increase the risk for stroke, blood clots and pregnancy complications.

Neanderthal DNA also appears to affect skin and hair colour, freckles, and even warts and calluses.

“We also found several surprising associations between Neanderthal DNA and psychiatric and neurological phenotypes (resulting behaviours) like depression and nicotine addiction,” Dr Capra said.

He quickly added that Neanderthals did not even have tobacco, which is native to the Americas. They probably were not depressed, either.

Dr Capra explained that “depression is an incredibly complex disease and we don’t fully understand the genetic and environmental drivers of depression today in modern populations.

“Thus, like nicotine addiction, depression might not even make sense as a ‘disease’ 50,000 years ago.

“Nonetheless, this does suggest a potential influence of Neanderthal DNA on related systems.”

Ongoing debates about human history and evolution

Reaction to the new study reflects ongoing debates about human history and evolution.

Some researchers, for example, believe that Neanderthals and humans were very different, with Neanderthals going extinct possibly at the hands of Homo sapiensabout 40,000 years ago. Still others believe that Neanderthals were simply absorbed into the Homo sapiens population.

There are also ongoing debates about “Out of Africa” and “Out of Asia” on where various human species originated and migrated to over the years.

One thing is clear: our family trees are far more complex than anthropologists from earlier generations had ever imagined.

Emeritus Professor Ian Tattersall of the American Museum of Natural History said that “any (DNA) admixture did little to affect the future evolutionary trajectory of either Homo neanderthalensis or Homo sapiens.

“Whether or not the conditions the authors discuss are based on Neanderthal-derived genes, they present identical issues to clinicians,” he said.

“Blaming certain forms of depression on Neanderthal genes doesn’t help us much in dealing with them.”

On the other hand, Dr Sriram Sankararaman, a geneticist at Harvard Medical School, said the key advance is researchers can now associate Neanderthal DNA, which appears to affect when and where nearby genes are turned on and off, with physical and health traits.

Dr Sankararaman said the association with “depression and mood disorders is new,” as is the “super-interesting” association with keratosis, the skin condition that can lead to warts, calluses, eczema and more.

This story was first published on Discovery.com