If you run a finger along the front of your jaw, you’ll feel a small bump just below your lower lip. It’s easy to overlook, but that tiny feature – the human chin – is something no other living primate has.
Chimpanzees, gorillas, and even our closest extinct relatives, such as Neanderthals, all lack a true chin.
For decades, scientists have debated why this distinctive feature evolved. Did the chin serve a specific biological purpose, or did it simply appear as a side effect of other changes in the human skull?
A new study offers one of the clearest answers yet, suggesting that the chin may not have been directly “designed” by natural selection at all.
The chin sets humans apart
A chin is an anterior bony projection of the lower jaw, also called the mandible. Only Homo sapiens has a true chin. Neanderthals did not have one, and other extinct hominin species also lacked this feature.
Some researchers define a true chin as an inverted-T-shaped structure. This structure includes a triangular projection at the base of the jaw, a vertical midline keel, and small depressions on either side. This exact combination appears only in modern humans.
Because of this, scientists often use the chin to identify modern human fossils. However, chin shape varies from person to person.
Some people have a strong forward projection, while others have a smaller one. That variation makes it harder to understand how the chin first evolved.
Testing how the chin evolved
The study was led by Noreen von Cramon-Taubadel from the Department of Anthropology at the University at Buffalo. The research team examined 532 adult skulls and mandibles from 15 living ape species.
Instead of assuming that natural selection shaped the chin directly, the researchers tested three possibilities. The chin might have formed through random genetic drift.
It might have evolved through direct selection. Or it might have formed as a byproduct of selection acting on other skull and jaw regions.
To explore these ideas, the team measured many cranial and mandibular traits. They then calculated how quickly each trait changed along different branches of the ape evolutionary tree.
If a trait changed faster than expected under random-drift, that pattern suggested directional selection.
Major changes in the human lineage
The results showed strong directional selection along the branch leading from the last common ancestor of humans and chimpanzees to modern humans. This finding means important skull and jaw changes occurred during human evolution.
Selection increased basicranial-flexion, meaning the base of the skull bent more. Selection also expanded the neurocranium, the part of the skull that houses the brain.
At the same time, selection reduced lower-face projection and reshaped the mandible into a more gracile and parabolic-form instead of the U-shaped jaw seen in apes.
These patterns match what scientists already know about early hominins. As ancestors adopted bipedal-walking, the skull balanced differently over the spine. The face became less projecting. The front teeth became smaller.
Changes in diet, including softer and cooked-foods, likely reduced chewing-demands. All of these shifts influenced the shape of the skull and jaw.
How the human chin evolved
The researchers focused on nine specific chin-related traits in the symphyseal-region of the mandible. If the chin evolved as a direct adaptation, most of these traits should show strong signs of selection.
However, the results told a different story. Only three of the nine chin-related traits showed evidence of direct-selection.
Three traits showed no significant-selection. The remaining three traits showed indirect-selection, meaning they changed because of their connection to other traits.
This pattern suggests that natural-selection did not act directly on the chin as a single feature. Instead, the chin likely changed as other parts of the skull and jaw evolved.
The chin as a spandrel
“The chin evolved largely by accident and not through direct selection, but as an evolutionary byproduct resulting from direct selection on other parts of the skull,” said von Cramon-Taubadel.
The study supports the idea that the chin is a spandrel. A spandrel is a feature that appears as a side effect of other changes. In architecture, spaces form between arches without anyone planning them. In biology, some traits appear because nearby traits change.
“While we do find some evidence of direct selection on parts of the human skull, we find that traits specific to the chin region better fit the spandrel model,” said von Cramon-Taubadel.
“The changes since our last common ancestor with chimpanzees are not because of natural selection on the chin itself but on selection of other parts of the jaw and skull.”
Integration explains the outcome
The study also highlights morphological integration. Skull and jaw traits are genetically linked and functionally connected.
When selection reshaped the braincase, reduced the face, and changed tooth size, the front of the mandible responded as part of an integrated system.
Reduction in anterior tooth size likely shortened the alveolar region of the mandible. At the same time, bone deposition in the basal region continued forward.
This combination gradually created a more upright symphyseal angle and eventually produced the projecting chin.
“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,” said von Cramon-Taubadel.
A small feature with a big lesson
The human chin may seem like a minor detail, but its history reveals an important lesson about evolution.
Not every unique-feature exists because natural-selection directly shaped it for survival. Some features arise as connected side-effects of broader structural-changes.
The chin likely formed as a byproduct of brain expansion, facial reduction, dental changes, and bipedal adaptation.
In that sense, this small bony projection offers powerful evidence that evolution works through complex, interconnected changes rather than simple one-purpose designs.
The study is published in the journal PLOS One.
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