Every human face is different, but scientists still know surprisingly little about how our DNA shapes these differences.
To learn more, researchers are turning to our ancient relatives, the Neanderthals, whose facial features were noticeably different from ours.
Neanderthals had large noses, strong brows and a bigger, more robust lower jaw.
Understanding what made their faces unique could help us uncover how human faces form and evolve.
A new study from the MRC Human Genetics Unit at the University of Edinburgh shows that Neanderthal DNA may hold important clues.
Published in the journal Development, the research reveals that a small region of Neanderthal DNA is better at activating a jaw-shaping gene than the same region in modern humans.
This difference may help explain why Neanderthals had larger lower jaws.
Hannah Long, who led the study, says that scientists have fully sequenced the Neanderthal genome using fragments of DNA taken from ancient bones.
Although Neanderthals are long extinct, their genome is extremely similar to ours—99.7 percent identical. This tiny fraction of differences includes genetic changes that likely contributed to their distinctive appearance.
However, finding exactly which genetic differences influence facial shape is extremely difficult. Both human and Neanderthal genomes consist of about 3 billion DNA letters, most of which are not involved in appearance.
Fortunately, Long and her team focused on a part of the genome already linked to Pierre Robin sequence, a condition where babies are born with unusually small lower jaws. People with this condition often have large deletions or rearrangements in this region of DNA, which disrupt normal jaw development.
The researchers suspected that smaller, more subtle differences could also affect jaw shape.
When they compared this region of the genome in humans and Neanderthals, they found only three single-letter differences in a stretch of about 3,000 letters.
Although this region does not contain any genes, it acts as a switch that controls how and when an important gene called SOX9 is turned on. SOX9 plays a major role in coordinating facial development in the embryo.
To test whether these tiny DNA changes mattered, the team used zebrafish embryos, which are often used to study development.
They inserted both the Neanderthal and human versions of the DNA region into the zebrafish and gave each version a different fluorescent color. As the embryos grew, the researchers could see exactly when and where each DNA switch became active. Both versions were active in cells that form the lower jaw, but the Neanderthal version was clearly more active.
This suggested that Neanderthal DNA could trigger stronger activation of SOX9. To test the effects, the researchers increased SOX9 levels in the zebrafish embryos. As expected, the cells that help shape the jaw expanded, hinting at how increased SOX9 activity might lead to a larger, stronger jaw—similar to what we see in Neanderthal fossils.
Long says her team now hopes to study additional DNA differences using lab models that mimic human facial development. Their work could eventually help improve diagnosis and understanding of facial conditions.
This study shows that even though Neanderthals disappeared thousands of years ago, their DNA still has a lot to teach us about how our own faces form and evolve.
