Most modern humans have a bit of Neanderthal hidden somewhere in their genes. For years, it was assumed that this small dose of DNA came from a brief encounter between our ancestors and their Neanderthal neighbors thousands of years ago.
But what started as a one-night adventure now looks more like a frequent love story. A new analysis of the modern human genome suggests that our ancestors did the writing with their Neanderthal cousins more than a few times throughout their shared history.
It's the classic on-again off-again relationship, but on an epic time scale. When the first humans left Africa and entered Europe and Asia, they met their first Neanderthals. And when two sexually compatible beings coexist for approximately 30,000 years, a small handkerchief is likely to occur.
Today, most people have about two percent Neanderthal DNA, a reminder of the sexual tendencies of our ancestors. In fact, the only people who do not have a bit of Neanderthal DNA are those whose ancestors stayed in Africa, and never rejoiced with their northern neighbors.
Recently, however, scientists have been looking more closely at the human genome, and have noticed something curious about Neanderthal DNA in particular. It seems that people in East Asia have a Neanderthal DNA that is 12 to 20 percent higher than in people of strictly European descent.
This discovery opened a new possibility. Instead of meeting only once, Neandertals and modern humans may have reproduced multiple times throughout their shared history. The explanation fits, but the investigation was missing.
With this in mind, two researchers from Temple University decided to follow some different research paths.
Using a large data set of modern human genomes, the researchers compared the patterns of Neanderthal DNA in people of East Asian and European descent. The results confirmed that both groups had early multiple pairing events with Neanderthals.
Then, the researchers used an automatic learning algorithm to devise all crossing events that could have led to the Neanderthal DNA patterns they were seeing.
The best models did not fit with the idea that Neandertals and modern humans had a single episode of miscegenation.
In contrast, the relationship between our ancestors and their Neanderthal cousins seems more complex than we appreciate. The findings suggest that there were frequent interactions between the two groups, and there are likely to be multiple sexual encounters between Neandertals and prehistoric humans in Europe and East Asia.
"Therefore, we believe that a probable explanation for our results is that the flow of genes between humans and Neandertals was intermittent and continuous, but in a somewhat geographically restricted region," the authors conclude.
Understanding the different percentages of Neanderthal DNA can offer us a glimpse into ancient human history. These discrepancies can tell us a lot about how long certain populations coexisted with Neandertals compared to others.
They also indicate how closely related modern humans are with Neandertals, something that scientists have long discussed.
If we want to know more about the complex history between Neanderthals and modern humans, scientists will only have to delve further into the human genome.
This study has been published in. Ecology of nature and evolution.