Skull studies shed light on interbreeding between humans and Neanderthals

Deep in the genome of modern humans lie 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 relative of the extinct human to today’s human beings (Gay spas). Multiple studies suggest that Neanderthals interbred with humans around 100,000 years ago, and a new study published yesterday in the journal Biology is based on our knowledge of where this crossing occurred.

“Ancient DNA has revolutionized the way we think about human evolution,” Stephen Churchill, co-author of the study and professor of evolutionary anthropology at Duke University, said in a press release. “We often think of evolution as the branches of a tree, and researchers have spent a lot of time trying to trace the path that led to us, Homo sapiens. But now we’re beginning to understand that it’s not a tree—rather, it’s 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 determine 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 examine the size and shape of key facial structures. Having a control allowed the team to best determine whether the 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) that are associated with changes in human facial features to determine the possible likelihood that connections between Neanderthal and human populations were the result of interbreeding rather than another factor.

“We found that the facial features we focused on were not strongly influenced by climate, which made it easier to identify likely genetic influences,” said Ann Ross, corresponding author of the study and professor of biological sciences at NC State University. “We also found that facial shape was a more useful variable for tracking the impact of Neanderthal interbreeding in human populations over time.” Neanderthals were simply bigger than humans. Over time, the size of human faces became smaller and 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 research is to take measurements from more human populations, such as the Natufian culture, which lived more than 11,000 years ago in the Mediterranean Sea in the present-day nations of Israel, Jordan and Syria. Their findings from comparing these skulls supports the hypothesis that much of this interbreeding occurred in a region ranging from North Africa to Iraq. “This was an exploratory study. And to be honest, I wasn’t sure this approach would really work – we have a relatively small sample size and 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,” Churchill added.

“The picture is really complex,” Churchill said. “We know there was cross-breeding. 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 suggests that Neanderthals interbred with what are now modern humans as our prehistoric ancestors left Africa but before spreading to Asia. Our aim with this study was to see what further 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 shelter, 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 Neanderthal genome was sequenced in 2010.

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