An exploratory study by North Carolina State University is offering new insights on the interbreeding between Neandertals, otherwise commonly known as Neanderthals, and modern humans, suggesting that most of the mating took place in the Near East, in the region ranging from North Africa to Iraq.
This new evidence shows how populations moved and interacted over time, proving that the Near East was an important crossroads, not only geographically but also in the context of human evolution.
The study used data on craniofacial morphology from the published literature, which resulted in a data set including 13 Neandertals, 233 prehistoric Homo sapiens, and 83 modern humans.
Anthropologists already knew there was interbreeding, and the goal of this study was to see what more they could learn by assessing the facial structure of prehistoric humans and Neandertals.
“Modern Asian populations seem to have more Neandertal 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 before spreading to Asia,” says Steven Churchill, co-author of the study and a professor of evolutionary anthropology at Duke University.
“I wasn’t sure this approach would 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 compelling,” he adds.
Facial characteristics reveal Neandertal interbreeding
The researchers focused on standard craniofacial measurements, which are reproducible, and used them to assess the size and shape of key facial structures in order to determine whether a given human population was likely to have interbred with Neandertal populations, and to what extent.
They also accounted for environmental variables associated with changes in human facial characteristics to determine the likelihood that connections they established between Neandertal and human populations were the result of interbreeding rather than other factors.
“Neandertals had big faces,” Churchill says. “But size alone doesn’t establish any genetic link between human and Neandertal populations. Our work here involved a more robust analysis of the facial structures.”
His team found that the facial characteristics they focused on were not strongly influenced by climate, which made it easier to identify likely genetic influences. They 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, 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,” explains Ann Ross, corresponding author of the study and professor of biological sciences at North Carolina State University.
Tracing back human evolution
Professor Churchill points out that people often think of human evolution as branches on a tree, and researchers have spent a lot of time trying to trace back the path that led to Homo sapiens -the modern day human.
“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,” he states.
According to Professor Ross, their work provides a deeper understanding of where those streams came together.
To build on these first results, their team would like to incorporate measurements from more human populations in the future, 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 concludes.