Ancient skulls shed light on human interbreeding

Traces of Neanderthal DNA in modern humans may be attributed to interbreeding that took place in the “Near East,” which ranges from North Africa to Iraq. This is the conclusion of a new study that analyzed the facial features of prehistoric skulls.

Study co-author Steven Churchill is a professor of evolutionary anthropology at Duke University. “Ancient DNA caused a revolution in how we think about human evolution,” said Professor Churchill. 

“We often think of evolution as branches on a tree, and researchers have spent a lot of time trying to trace back the path that led to us, Homo sapiens. But we’re now beginning to understand that it isn’t a tree – it’s more like a series of streams that converge and diverge at multiple points.”

“Our work here gives us a deeper understanding of where those streams came together,” explained study co-author Professor Ann Ross of North Carolina State University.

“The picture is really complicated,” said Professor Churchill. “We know there was interbreeding. Modern Asian populations seem to have more Neanderthal DNA than modern European populations, which is weird – because Neandertals lived in what is now Europe. That has suggested that Neandertals interbred with what are now modern humans as our prehistoric ancestors left Africa, but before spreading to Asia.” 

“Our goal with this study was to see what additional light we could shed on this by assessing the facial structure of prehistoric humans and Neandertals.”

According to Professor Ross, facial morphology allows us to trace the intentions and movements of human populations over time. “And the evidence shows us that the Near East was an important crossroads, both geographically and in the context of human evolution.”

For the investigation, the researchers analyzed data from published studies involving 13 Neandertals, 233 prehistoric Homo sapiens, and 83 modern humans.

After assessing the size and shape of key facial structures found in the skulls, the experts conducted an in-depth analysis to determine the likelihood that a given human population interbred with Neanderthal populations.

“Neanderthals had big faces,” said Professor Churchill. “But size alone doesn’t establish any genetic link between a human population and Neandertal populations. Our work here involved a more robust analysis of the facial structures.”

The experts accounted for environmental variables that may have altered facial characteristics in the skulls. 

“We found that the facial characteristics we focused on were not strongly influenced by climate, which made it easier to identify likely genetic influences,” said Professor Ross. “We also found that facial shape was a more useful variable for tracking the influence of Neandertal interbreeding in human populations over time. Neandertals were just bigger than humans. Over time, the size of human faces became smaller, generations after they had bred with Neandertals. But the actual shape of some facial features retained evidence of interbreeding with Neandertals.”

“This was an exploratory study,” said Professor Churchill. “And, honestly, I wasn’t sure 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, ultimately, the results we got are really compelling.

“To build on this, we’d like to incorporate measurements from more human populations, such as the Natufians, who lived more than 11,000 years ago on the Mediterranean in what is now Israel, Jordan and Syria.”

The study is published in the journal Biology.

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By Chrissy Sexton, Earth.com Staff Writer