UNLESS creatures such as yeti and Bigfoot turn out to be real, the only kind of human in the modern world is Homo sapiens. But that is only recently true. For most of Homo sapiens’s 200,000-year history, it shared the planet with several cousins. The most famous were the Neanderthals, who were larger and heavier and who lived in Europe and Central Asia.

Neanderthals died out 40,000 years ago. Whether they were killed off directly by modern humans or were out-competed is a perennial topic of debate. But, either way, there was more to relations between the two species than just inter-hominid rivalry. In the past decade researchers have found that between 1% and 4% of the DNA of modern Europeans and their descendants on other continents is of Neanderthal origin. There must, in other words, have been a certain amount of interbreeding going on back in the day.

What is less clear is the effect today of the DNA so acquired. Neanderthal DNA is not easy to come by (it must be garnered from fossils that have been particularly well preserved), and doing extensive genetic testing on large numbers of modern humans, which is necessary to untangle the influence of even a relatively small chunk of their genomes, is expensive. Nevertheless, as they report in Science, exactly such a comparison has just been made by a team lead by Corinne Simonti of the Vanderbilt Genetics Institute, in Tennessee.

Rather than doing the genetic tests themselves, Dr Simonti and her colleagues used data from the Electronic Medical Records and Genomics Network. This gave them both the medical histories and the genotypes of thousands of people. They picked out 28,416 people of European descent and compared the genomes of these individuals with genetic information recovered from the toe-bone of a Neanderthal woman that was found in a cave in Russia, in 2010. They found 135,000 bits of modern human DNA which they thought were probably of Neanderthal origin.

Previous research had found such Neanderthal DNA to be especially common near parts of the genome associated with illnesses like depression, heart disease and seborrheic keratosis, a complaint in which scaly lumps form on a sufferer’s skin. Because Dr Simonti’s data included people who actually suffer from such conditions, she was able to check those associations. When she did so, she found that particular chunks of Neanderthal DNA were indeed correlated with the presence of all three complaints. And the team found a clutch of other phenomena for which Neanderthal genes seemed to put their carriers at additional risk. These ranged from obesity and blood-clotting disorders to certain types of malnutrition, and even smoking.

At first blush this seems to suggest Neanderthal DNA is a curse. But that is almost certainly not the case. Forty millennia is plenty of time for evolution to get to work. This means that unfavourable traits should have been weeded out, while beneficial ones spread. There is evidence of exactly this. Some parts of the human genome are unusually free from Neanderthal influence, suggesting natural selection has removed harmful genes. Other parts, where Neanderthal DNA presumably offers benefits, are full of the stuff.

And just because something is bad for modern humans does not necessarily mean it was bad for their hunting-and-gathering ancestors. Some genes might put their bearers at risk of obesity in the modern world of fatty, sugary snacks. But in a world where food is scarce (as it presumably was in the northern latitudes where modern humans and Neanderthals mixed), those same genes might help their owners through lean periods.

Neanderthal DNA seems to put modern humans at risk of a specific sort of malnutrition caused by a lack of thiamine, a B vitamin that is vital for carbohydrate metabolism. But, says Dr Simonti, that same genetic variant may also make it easier to digest fats. Millennia ago, when people obtained less of their energy from refined carbohydrates, the trade-off may have been worthwhile. In a world where grain crops have become a staple food, it may not be.

Finally, and most prosaically, Dr Simonti and her colleagues were looking at medical records. These, by definition, will contain information only about diseases. Evolution is often a trade-off, and for Neanderthal DNA to have survived as long as it has, it must offer some advantages to those carrying it. For the time being, exactly what those advantages are is a mystery. But more research will surely illuminate them.