Only Humans Have Chins — and They Emerged as an Evolutionary Accident
Most components of the human body have been driven by natural selection to ensure our species’ survival — the chin, though, is not one of them. Chins are uniquely human, since no other animals have them, not even chimpanzees, our closest relatives. It might seem that they would grant some sort of advantage that only we have, but research on the evolution of chins says otherwise.
A new study published in PLOS One has shown that the chin may have emerged by pure coincidence. Rather than natural selection driving the development, the chin developed in modern humans as an accidental byproduct following long-term evolution of the jaw in early hominins. As a result, the chin wasn’t really designed for a true purpose, shaking up conventional views of humans’ physical characteristics.
“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 study author Noreen von Cramon-Taubadel, a professor of anthropology at the University at Buffalo, in a statement. “The chin is likely a byproduct, not an adaptation.”
The Chin as an Evolutionary Accident
Human faces wouldn’t look complete without a chin, anatomically defined as a bony protrusion that extends from the lower jaw. Previous studies suggested that chins evolved in humans as a way to resist biomechanical stresses related to mastication, or in simpler words, chewing.
However, more recent research has begun to lean toward the idea that the chin didn’t arise as an adaptation to boost survival chances. The new study contends that the chin is a spandrel, a term used to describe a feature in an organism that is unintentionally produced by other adaptations. The term was popularized by American evolutionary biologist Stephen Jay Gould, who noticed that the triangular spaces (or spandrels) between the top curve of an arch serve no architectural purpose. Instead, they’re byproducts of the arch itself.
In the past, the scientific community believed that every feature of an animal that conferred a positive effect must have been achieved by natural selection. This view has since been challenged by new research.
Read More: Modern Humans May Have Lived Alongside an Extinct Human Species in Ancient Indonesia
Comparing Primate Faces
The researchers involved with the new study determined how the chin evolved by comparing cranial traits of apes and humans. They focused on nine “mandibular” (chin) traits, finding that only three were under significant direct selection, while the remaining six were either under indirect selection or no selection.
“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 in the release. “The changes since our last common ancestor with chimpanzee are not because of natural selection on the chin itself but on selection of other parts of the jaw and skull.”
According to the researchers, the chin likely developed from a combination of factors, including the decrease in size of the lower jaw and teeth in early hominins. Since the chin only appeared once modern humans came along, some of the direct selection pressures that resulted in these changes must have persisted for most of hominin evolution, the researchers wrote in the study.
The reduction in size of the lower face, for example, may have been a long-term response to shrinking teeth (namely incisors and canines) in hominins over time.
Challenging Traditional Views
Although the chin didn’t specifically evolve to serve a survival function, it still helps us by reinforcing the lower jaw’s structure. Still, the researchers say it’s important to consider that natural selection did not directly prompt all physical characteristics.
“Generating empirical evidence against that line of reasoning is an important goal of this study and biological anthropology in general,” said von Cramon-Taubadel. “The findings underscore the importance of assessing the evolution of physical characteristics with trait integration in mind.”
Read More: How Burn Injuries May Have Shaped Human Evolution, and Set Us Apart From Other Primates
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