If you have some European and Asian ancestry, chances are, your ancestors had sex with Neanderthals (not even mentioning our affairs with the Denisovans). It turns out, these close encounters were much more than just a one night stand.

A fresh analysis of genetic data has suggested that Neanderthals and modern humans had numerous episodes of interbreeding through their relatively brief time together in Eurasia. This is an idea that’s been raised before, but now further evidence has been added. If the new study's conclusions are on the money, it means that the story of humans and Neanderthals is a lot more complex and fiddly than we previously thought, just as affairs of the heart often are.

Earlier research hinted that the Neanderthal genes were introduced into humans during a single period of intermingling. This would have occurred sometime after modern humans hiked out of Africa into Eurasia and came into contact with Neanderthals some 75,000 years ago, but before the Neanderthals went extinct around 40,000 years ago.

We know of these sexual encounters because the Neanderthal lineage makes up a small but significant chunk, between 2 and 6 percent, of the DNA in almost every human with ancestry outside of sub-Saharan Africa today. However, more recent work has shown that modern East Asian populations carry significantly more Neanderthal DNA than European populations.

As reported in the journal Nature Ecology & Evolution, this is further evidence in support of the theory that there were multiple and prolonged encounters.

The researchers looked at the distribution of Neanderthal chunks across genomes in the 1,000 Genomes Project, a vast catalog of human genetic variation. Through modeled simulations, they found that the percent of Neanderthal DNA in contemporary Europeans and East Asians is too high to account for just a single encounter. 

“We… found strong support for modern humans interbreeding with Neandertals multiple times, first and foremost in the Middle East, but also later in both East Asia and Europe,” study author Fernando Villanea, a molecular anthropologist from Temple University in Philadelphia, explains in a "behind-the-paper" article in Nature.

“The secondary admixture in East Asia was ever so slightly more extensive, resulting in the elevated contribution to the genomes of living East Asian individuals we see today.”

The aftermath of this strange love story can still be felt today. Other studies have shown that Neanderthal gene variants can account for a whole number of traits that are still expressed in modern humans. For example, there are links between Neanderthal DNA and depression, obesity, and certain skin disorders. Some of these genes have even led humans to have a genetic propensity for nicotine addiction.