There are traces of Neandertal DNA in the genome of modern humans, according to research. Now, an exploratory study that examined the facial structure of prehistoric skulls provides new information and supports the hypothesis that much of this interbreeding occurred in the Near East, which stretches from North Africa to Iraq.
“Ancient DNA caused a revolution in how we think about human evolution,” says co-author and Duke University professor of evolutionary anthropology Steven Churchill. “We often think of evolution as a tree with branches, and scientists 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,” says Ann Ross, corresponding author of the study and a professor of biological sciences at North Carolina State University.
We discovered that the facial characteristics we focused on were not strongly influenced by climate. This made it easier to identify likely genetic influences. In addition, we discovered that facial shape was a more useful variable for tracking the impact of Neandertal interbreeding in human populations over time.
Ann Ross
Neanderthal-derived DNA has been discovered in the genomes of most, if not all, modern populations, with the amount varying noticeably by region. It accounts for 1-4% of modern genomes for people living outside Sub-Saharan Africa, though estimates vary, and none or up to 0.3% for those living in Africa, according to recent research. East Asians have the highest level, Europeans have a medium level, and Southeast Asians have the lowest level.
“The situation is extremely complicated,” Churchill says. “We’re aware of interbreeding. Modern Asian populations appear to have more Neandertal DNA than modern European populations, which is strange given that Neandertals lived in Europe. This suggests that Neandertals interbred with modern humans after our 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.”
“By evaluating facial morphology, we can trace how populations moved and interacted over time,” Ross explains. “And the evidence shows us that the Near East was an important crossroads, both geographically and in the context of human evolution.”
For this study, the researchers collected data on craniofacial morphology from the published literature. This ultimately resulted in a data set including 13 Neandertals, 233 prehistoric Homo sapiens, and 83 modern humans.
The researchers concentrated on reproducible standard craniofacial measurements and used those measurements to assess the size and shape of key facial structures. This allowed the researchers to conduct a more in-depth analysis to determine whether a specific human population was likely to have interbred with Neandertal populations, as well as the extent of that likely interbreeding.
Churchill describes Neandertals as having “big faces.” “However, size alone does not establish a genetic link between human and Neandertal populations. Our work here entailed a more thorough examination of the facial structures.”
The researchers also accounted for environmental variables that are 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.
“We discovered that the facial characteristics we focused on were not strongly influenced by climate,” Ross says. “This made it easier to identify likely genetic influences. In addition, we discovered that facial shape was a more useful variable for tracking the impact of Neandertal interbreeding in human populations over time. Neandertals were simply larger than humans. After humans bred with Neandertals, the size of their faces shrank over generations. However, the shape of some facial features retained evidence of Neandertal interbreeding.”
“This was a preliminary study,” Churchill explains. “And, to be honest, I wasn’t sure this approach would work because we had a small sample size and didn’t have as much data on facial structures as we would have liked.” But, in the end, the results are very compelling.” To build on this, we’d like to include data from other human populations, such as the Natufians, who lived on the Mediterranean in what is now Israel, Jordan, and Syria more than 11,000 years ago.”
