Neanderthals may have died out tens of thousands of years ago, but their DNA still influences modern humans, according to a study published in Science today. Researchers from Vanderbilt University have confirmed a small but tangible link between Neanderthal DNA inherited by our interbreeding ancestors, and wide range of traits from blood clotting to depression. The effect is slight (the presence of Neanderthal DNA only affects 1 percent of a person's risk of depression, for example), but significant, showing the lasting legacy of these ancient hookups.

Despite the small impact this Neanderthal DNA has on us today, it’s possible that it was very useful tens of thousands of years ago. By the time our ancestors arrived in the Eurasian continent, Neanderthals had been living there for hundreds of thousands of years, and had adapted to the colder climate. It’s likely that breeding between the two species allowed our ancestors to hijack the genetic advantages Neanderthals had developed over time to cope with their environment. "Perhaps spending a night or two with a Neanderthal was a relatively small price to pay for getting thousands of years of adaptations," said John Capra, the study's senior author, at a press conference.

These adaptations might have included changes to the skin. When the presence of this Neanderthal DNA was first discovered in the human genome, it was often showed up near genes for making keratin, a protein found in our nails, hair, and skin. This new study shows that the presence of certain Neanderthal genetic variants is associated with the development of actinic kerastoses — a condition where exposure to the Sun causes patches of dry, scaly skin. This suggests that although the Neanderthal variants that were inherited were useful 50,000 years ago, they're not so helpful now.

Another condition that's more likely to occur when an individual possesses certain bits of Neanderthal DNA is increased blood coagulation. It's thought that this trait might have been useful for our scrappier ancestors, helping keep wounds free from the Eurasian pathogens that their bodies had not encountered before. However, in modern humans, it's more likely to increase the risk of strokes.

The study was able to prove the effects of Neanderthal DNA thanks to a large database of linked genetic data and medical records known as eMERGE (the the Electronic Medical Records and Genomics Network). Using eMERGE, researchers were able to cross-reference the presence of traits and diseases with the appearance of known Neanderthal genetic variants. They studied anonymized data from a total of 28,416 adults of European ancestry living in the US. "Having both disease information and Neanderthal DNA present in these individuals enabled us to test for relationships between the two," said doctoral student Corinne Simonti, the first author of the paper.

Although the effect of Neanderthal DNA on our skin had been hypothesized, the connection with depression and mental disorders was less expected. Especially a link between Neanderthal DNA and nicotine addiction — after all, nicotine comes primarily from tobacco, a plant that didn't exist in Eurasia at this point in time. Our ancestors weren't "walking around, puffing on cigarettes outside the cave," Capra says. He adds that while we can speculate that nicotine addiction might be an instance of some larger neurological trait — a tendency toward addiction in general, for example — this particular connection only proves how difficult it is to explain causation when it comes to ancient fragments of DNA.