Human beings are the the only primates to have a real chinbut what is it for and why are we the only ones with it? In a study released this year on PLOS OneDoctor Noreen von Cramon-Taubadel and her team tried to verify this very point. The data they report goes in an unexpected direction: the chin does not appear to have been directly selected. More likely it appeared as a result of other evolutionary changes which involved the skull, teeth and jaw along the line that leads to our species. In short, by chance. The research team examined 532 skulls and jaws belonging to 15 hominoid species. By comparing precise measurements between different anatomical points, he reconstructed how they have changed over time the various segments of the face. The picture that emerges is interesting: most of the characters linked to the chin do not seem to have evolved because they were “pushed” directly by natural selection, but because linked to other traits that were changing to lead to the appearance ofHomo sapiens.
What exactly is a chin
If we want to put it simply, the chin is that portion of bone that protrudes forward in the lower part of the jaw, immediately under the lower incisors. From an anatomical point of view it is located in the so-called mandibular symphysis: it is the central line where, during growth, the two halves of the jaw join together to become a single bone. However, a slight protrusion is not enough to speak of a “real chin”. Some authors define it as a inverted T-shaped structure composed of: a triangular prominence at the base (mental trigonum), a central vertical ridge, two small lateral depressions. This complete combination it is typical ofHomo sapiens.
Genetics, functionality or chance? Hypotheses on the appearance of the chin
Over time, many explanations have been proposed, for example the resistance to the forces of chewing, the effect of the tongue and speech muscles or the signal of sexual attractiveness. The study published in January 2026 aimed to put it to the test three scenarios:
- Genetic derivation: random changes accumulated over time.
- Direct selection: the chin would have been favored because it was useful in itself.
- Indirect selection: the chin would be a “side effect” of other transformations.
To do this, the researchers used an approach of quantitative genetics. In practice, instead of treating the chin as a single “present or absent” feature, they measured dozens of distances between anatomical points of the skull and jaw.
A sample of 532 adult individuals: the study
The sample included 532 adult individualsbelonging to 15 hominoid taxa. 32 anatomical points (15 cranial, 17 mandibular) and 46 distances between these points were recorded, with a measurement error of less than 1 millimeter.
The data was analyzed along a complete evolutionary tree of hominoids. To estimate the speed by which traits changed over time, a measure called Lande’s generalized genetic distance (GGD). Simply put, this measure compares how much two species differ in the form of a certain parameter, how much time has passed since their separation, how large the population was. If the change is greater than expected compared to a random pattern, it is called directional selection: a certain configuration is systematically favored, so change accelerates in that direction. If it is less, it is called stabilizing selection, that is, it tends to maintain a stable shape, eliminating excessively extreme deviations and maintaining a certain structure.
What emerged from the study
In most evolutionary branches of the great apes, the shape of the skull and jaw is changed slowly or remained stable. The case of the branch that leads from the common ancestor of man and chimpanzee up to Homo sapiens: here there has been a lot of change faster than expectedwhich excludes the first hypothesisthat is, that it is only due to genetic drift.
A question therefore remains: did selection act directly on the chin or did the chin appear as a consequence of other changes? To answer, the researchers analyzed nine mandibular tracts connected to the symphysisi.e. to the chin region. Only three show signs of direct selection, while the others show no significant selection or change indirectly. This does not support the idea of a chin that evolved as an autonomous adaptation. Direct selection has in fact mainly concerned other aspects of the skull and jaw such as greater flexion of the skull base, expansion of the part that contains the brain, reduction of the lower face and the transformation of the jaw from a U shape, typical of monkeys, to a more parabolic and less robust shape. Everyone changes consistent with bipedalism and with the progressive reduction of the anterior dentition in the first hominins.
Therefore, only the hypothesis remains indirect selection. To describe the chin, the authors use the term “spandrel”taken from architecture: indicates a space that is born as effect of the overall structurenot because it was designed with a specific purpose. In biology it means that a trait can emerge as by-product of other adaptations. In the human case, the reduction of the alveolar region, i.e. the part of the jaw that contains the teeth, together with the different growth of the basal part, has made the symphysis more vertical and progressively protruding. This result fits into the framework ofmorphological integration: skull and jaw are connected systems, and when some regions change under selection, others change accordingly. The human chin therefore represents the outcome of these coordinated transformations along the evolutionary line which led to Homo sapiens.
