Male nipples. Whale pelvic bones. Vestigial hind limbs in snakes. Evolution is full of features that look purposeful. But upon closer inspection, science tells us that they are actually by-products of development and shifting gene expression. New research suggests the human chin may be one such evolutionary spandrel, a term biologists use for a trait that emerges as a consequence of other changes rather than as a direct adaptation.
For decades, scientists have assumed the human chin must serve some adaptive purpose. After all, no other primate developed the forward-projecting lower jaw that defines Homo sapiens. That anatomical exclusivity has made the chin one of the most distinctive traits of our species, and a persistent evolutionary puzzle.
To date, researchers have proposed a range of functional explanations, from reinforcing the jaw against chewing stress to supporting the muscular demands of speech or signaling mate quality. But instead of assuming the chin was shaped by natural selection for a specific role, a team led by biological anthropologist Noreen von Cramon-Taubadel at the University at Buffalo tested a more basic possibility: what if the chin itself was never the target of selection at all?
To investigate, the team analyzed measurements from 532 crania and mandibles across 15 hominoid taxa, comparing patterns of morphological change along the branch leading from the last common ancestor of humans and chimpanzees to modern humans. Using a quantitative genetic framework, the researchers tested whether chin-related traits evolved faster or slower than expected under neutral genetic drift, and whether any detected selection acted directly on the chin or indirectly through changes elsewhere in the skull.
The human lineage showed evidence that natural selection was actively shaping the skull and lower jaw. Traits associated with smaller faces, reduced front teeth, widening of the back portion of the lower jaw, and changes in the angle at the base of the skull evolved more rapidly than expected under a neutral model of genetic drift.
To determine whether the chin followed that same pattern, the researchers examined it separately from the rest of the skull. When they analyzed nine specific measurements tied directly to chin shape and projection, the results told a different story. Only three showed patterns consistent with direct selection. The remaining traits showed no clear evidence of direct selection and instead changed indirectly, likely in response to selection pressures acting elsewhere in the skull.
The clearest signals of direct selection appeared not in the chin itself, but in broader changes to the skull and jaw. Over millions of years, the lower face became smaller and less projecting, while the braincase expanded. As these shifts reshaped the skull, the geometry of the lower jaw shifted accordingly. Bone growth at the base of the mandible, combined with reduced forward projection of the tooth-bearing region, gradually produced the prominence we recognize today as the chin.
In other words, the chin may not have been the driver of change, but the passenger. A conclusion that pushes back on a familiar evolutionary instinct: if a trait stands out, it must have been directly selected.
“Just because we have a unique feature, like the chin, does not mean that it was shaped by natural selection to enhance an animal’s survivability, for example, a buttress for the lower jaw to help dissipate the forces of chewing,” said von Cramon-Taubadel.
“The chin is likely a byproduct, not an adaptation. It’s only by studying the whole that we can better understand what aspects of an animal have a functional purpose and what are the side products of that purpose.”
Instead, von Cramon-Taubadel suggests these findings, published in the journal PLOS One, highlight the role of trait integration, the idea that anatomical features evolve as interconnected systems. When selection reshapes one region, other structures can shift in response because they are developmentally and genetically linked.
That does not mean, however, that the chin lacks function. Current research has shown that it provides structural reinforcement at the front of the lower jaw and may even help distribute muscular forces generated during chewing and speech. But the study also suggests those functional benefits likely followed the broader reshaping of the skull rather than driving the chin’s evolution in the first place.
Instead of being carved out for a specific purpose, the chin may have emerged as a structural consequence of evolutionary changes that reduced our faces and expanded our brains. In evolutionary biology, a feature’s distinctiveness does not necessarily mean it was directly selected. Sometimes when natural selection reshapes one part of the skull, other features emerge because the structure has changed.
The next steps for the team involve extending this analysis to fossil hominin specimens to better understand when these facial and jaw changes first began taking shape. By applying the same framework to fossil material, they hope to clarify when the modern chin emerged and how it developed alongside the broader restructuring of the skull. That work could help disentangle which traits were directly favored by natural selection and which arose as a structural consequence of deeper anatomical change.