The Human Chin May Be an Evolutionary Accident, Not a Design
Dashiell Hammett famously opened The Maltese Falcon by drawing attention to Sam Spade's jutting chin. It was one of several features used to sketch the detective's sharp profile. Yet, from an evolutionary standpoint, emphasizing the chin may have been unintentionally repetitive—because every human chin is distinctive. Indeed, humans are the only primates to possess one at all.
A Feature Unique to Homo sapiens
Our closest living relatives, chimpanzees, lack chins entirely. So too did Neanderthals, Denisovans and every other extinct human lineage. The ability to quite literally "take it on the chin" is uniquely ours. This singular trait makes the chin a defining marker of Homo sapiens in the fossil record.
Put simply, a chin is the forward projection of the lower jawbone. But why does it exist? What evolutionary forces shaped its emergence?
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New Study Questions the Purpose of the Chin
A study published in PLOS One, lead by a biological anthropologist at the University at Buffalo, suggests that the human chin may not have evolved for any specific purpose at all. Instead, the findings contribute to a broader understanding of the human body as a complex blend of adaptive traits and incidental evolutionary byproducts.
Evolution by Chance, Not Selection
"The chin evolved largely by chance rather than through direct selection," explained Noreen von Cramon-Taubadel, PhD, Professor and Chair of the University at Buffalo's Department of Anthropology within the College of Arts and Sciences.
"It appears to be an evolutionary byproduct arising from selection acting on other regions of the skull."
The Chin as an Evolutionary 'Spandrel'
In evolutionary biology, such a feature is known as a "spandrel"—a trait that emerges unintentionally. Much like the empty space beneath a staircase, which exists not by design but as a consequence of construction, the chin may simply be the architectural side-effect of other structural changes in the human skull.
The concept of a "spandrel", first popularized by evolutionary biologist Stephen Jay Gould, was inspired by the triangular spaces formed when arches were constructed to support the dome of St Mark's Basilica in Venice. These spaces serve no architectural function of their own; they simply exist as an inevitable consequence of the arches above them.
The same principle, researchers suggest, applies to the human chin.
Why the Chin Is Unlikely to Be an Adaptation
"Possessing a unique trait such as the chin does not automatically mean it evolved through natural selection to improve survival," explained Professor Noreen von Cramon-Taubadel.
"It is unlikely to function as a structural reinforcement for the lower jaw or to dissipate the forces of chewing. The chin is more plausibly a secondary structure rather than an adaptation.
"It is only by examining the organism as a whole that we can distinguish which traits serve a functional role and which arise as secondary consequences," Professor von Cramon-Taubadel explained.
Testing the 'Null Hypothesis' of Neutral Evolution
Although her team is not the first to propose that the chin represents a spandrel, their approach departs from earlier studies that largely assumed natural selection directly shaped the lower jaw.
Instead, the researchers tested what is known as the null hypothesis of neutrality:
- Cranial characteristics were compared between apes and humans
- The study examined whether chin formation occurred without targeted selection
- Evolutionary changes were analyzed across multiple regions of the skull
"While we identified evidence of direct selection in certain regions of the human skull, the traits specific to the chin align more closely with the spandrel model," she noted.
"The changes that occurred after our divergence from chimpanzees were likely due to selection acting on other areas of the skull and jaw—not on the chin itself."
Rethinking Adaptation in Human Evolution
Within anthropology, there has long been an adaptationist tendency to interpret physical traits as purposeful outcomes of natural selection. Differences observed between species often encourage the assumption that every characteristic has been deliberately shaped to serve a specific function.
"One of the central aims of this research—and of biological anthropology more broadly—is to provide empirical evidence that challenges that assumption," Professor von Cramon-Taubadel explained.
"Our findings highlight the importance of examining how traits are integrated with one another when evaluating their evolutionary history."
Why This Matters Beyond Anthropology
Understanding how traits like the chin emerged reshapes how scientists interpret:
- Human evolutionary history
- Fossil identification and classification
- The balance between adaptation and chance in biology
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