Deep within the genome of modern humans are traces of the DNA of a long-lost relative: the Neanderthal (homo neanderthalensis). They lived about 400,000 to 40,000 years ago and are the closest extinct human relatives to modern human beings (Spas for homosexuals). A body of research has shown that Neanderthals interbred with humans around 100,000 years ago, and a new study published yesterday in the journal Biology it is building on our knowledge of where this interbreeding took place.

“Ancient DNA revolutionized the way we think about human evolution,” said Steven Churchill, co-author of the study and a professor of evolutionary anthropology at Duke University in a press release. “We often think of evolution as branches on a tree, and researchers have spent a lot of time trying to trace the path that led to us, Homo sapiens. But we’re now starting to realize that it’s not a tree – it’s more like a series of streams that converge and diverge at multiple points.”

The team of researchers from North Carolina State University, Duke University and the University of the Witwatersrand in South Africa compiled previously published data on Neanderthal craniofacial morphology, or facial structure. Neanderthals had larger faces than modern humans, but facial size is not sufficient to establish a genetic link between them and human populations.

A dataset consisting of 13 Neanderthals, 233 prehistoric Homo sapiens and 83 modern humans was constructed by the team from the available literature. They focused on standard skull measurements as a control to study the size and shape of major facial structures. Having the control allowed the team to better determine whether a human population was likely to have interbred with Neanderthal populations and the extent of interbreeding.

[Related: Ancient milk-drinkers were just fine with their lactose intolerance–until famine struck.]

The researchers also used environmental variables (such as climate) associated with changes in human facial characteristics to determine the likelihood that connections made between Neanderthal populations and humans were the result of interbreeding rather than another factor.

“We found that the facial features we focused on were not strongly affected by climate, which made it easier to identify potential genetic influences,” said Ann Ross, corresponding author of the study and professor of biological sciences at Washington State University. North Carolina. “We also found that facial shape was a more useful variable for tracking the impact of Neanderthal interbreeding on human populations over time. Neanderthals were larger than humans. Over time, the size of human faces became smaller, generations after they interbred with Neanderthals. But the actual shape of some facial features retained evidence of interbreeding with Neanderthals.

[Related: The debate over ‘Dragon Man’ shows that human origins are still kind of messy]

The next step for this type of study is to take measurements from more human populations, such as the Natufian culture that lived more than 11,000 years ago in the Mediterranean Sea in what are now the nations of Israel, Jordan and Syria. Their findings from comparing these skulls supports the hypothesis that most of this interbreeding took place in a region ranging from North Africa to Iraq. “This was an exploratory study. And, frankly, I wasn’t sure that this approach would actually work—we have a relatively small sample size, and we didn’t have as much data on facial structures as we would have liked. But, at the end of the day, the results we got are really convincing,” added Churchill.

“Photography is really complicated,” Churchill said. “We know there was interference. Modern Asian populations seem to have more Neanderthal DNA than modern European populations, which is strange – because Neanderthals lived in what is now Europe. This has suggested that Neanderthals interbred with what are now modern humans after our prehistoric ancestors left Africa, but before they spread to Asia. Our aim with this study was to see what additional light we could shed on this by assessing the facial structure of prehistoric humans and Neanderthals.

Neanderthals are known for making and using a wide range of sophisticated tools, controlling fire, living in shelters, making and wearing clothing, hunting large animals and also eating plants. There is also evidence that they buried their dead, a sign of sophistication for the species. The first complete genome of a Neanderthal was sequenced in 2010.

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