This article was originally published on Sept. 14, 2022.
The discovery of the first Neanderthal fossil in 1856 immediately raised questions about their relationship to us Homo sapiens. It would be some time before solid DNA evidence came out to suggest we interbred with them, though pre-existing skeletal analyses had suggested the possibility. Modern DNA evidence tells us that not only was there human-neanderthal interbreeding, but a substantial amount of it.
Nailing down where this was happening has been tricky, and the DNA evidence has not been entirely helpful in this regard. The problem is that anatomically-modern humans had a long time period, and spanned a large area, where interbreeding could have taken place with Neanderthals in Europe. While that would suggest there should be a higher proportion of Neanderthal genes in European populations, the DNA evidence shows Asians have a higher proportion.
To help nail this down, forensic anthropologist Ann Ross of North Carolina State University and graduate student Kamryn Keys partnered with paleoanthropologist Steven Churchill of Duke University to apply a technique little-used in paleoanthropology: facial morphology analysis. Their results not only point to an answer to this mystery, but to a way of harvesting new data and drawing new insights from a limited fossil record.
Evidence of Human-Neanderthal Interbreeding: Surprising Results
The outcome, Churchill admits, far exceeded his initial expectations: “I was like, ‘there’s no way this is going to work,’ at the outset.” There were two basic reasons for this. First, the sample sizes they were working with were very small. Second, existing and well-defined facial measurements don’t perfectly capture the oddities of Neanderthal faces, particularly the 3D aspects like facial depth.
“We went into it recognizing it’s exploratory: We’re not testing hypotheses, just looking for pattern recognition,” he says. “And, lo and behold, the results were like, wow!”
Their findings indicated — as previous ancient DNA evidence had suggested — that the Near East was an important place for hybridization between early modern humans and Neanderthals. This region, currently around the Mediterraean shores, was known to play host to a long-lasting and well-defined boundary between humans and Neanderthals, but whether it was a region of genetic exchange was not known. An existing theory to explain the oddness of the DNA evidence available had been that the Near East had been a region of human-neanderthal interbreeding, with those humans carrying Neanderthal DNA into Asia as they expanded into it, which would account for the higher rates there than in Europe.
Ross says that analyzing facial morphology allows for a level of granular insight that’s very difficult to replicate with DNA: “It added a different perspective.”
The role of DNA analysis in paleoanthropology was transformative, she says. “DNA changed the discipline. We found Neanderthals definitely interbred with Denisovans” she says, as an example. “But through DNA you don’t get to really examine the actual patterns: Is it temporally related? Is it related to the climatic zones? Can you pinpoint a region?” Where facial analysis has been used in paleoanthropology, she adds, it has largely been qualitative, not quantitate. That is, descriptions have been included, but not measurements.
The Facial Morphology Process and Results
Their basic approach was to geographically subdivide their fossils by region, then see how similar they were to each other. “If there was no interbreeding whatsoever, what we would expect is that the data would segregate into two broad clusters. There would be Neanderthals on one branch, and early modern humans on the other branch,” Churchill says. “And we would expect to see some geographic structure in those branches,” with samples closer in geographic space looking more similar.
What they found was that their 100,000-year-old samples of modern humans in the Near East were exactly as similar to other human samples as they were to Neanderthals. “And that’s what you would expect if there was hybridization,” he adds.
Churchill cautions that the findings are not watertight, due to the small sample sizes in this study. “I wouldn’t take this to the bank and say we’ve pinned it down. But it’s interesting that it’s sort of consistent with the DNA that points to the Near East as a major locus,” he says.
And while the results are exciting, so too are the methods themselves.
“People have used facial morphology, as well as other details of the cranium and teeth, to say this [fossil] looks Neanderthal-like, though it’s clearly a modern human," Churchill says. "But this was, to my knowledge, the first ever to look at … large samples of early modern humans, rather than just single fossils, to say it looks Neanderthal-like.”