Only humans have chins. It was an evolutionary accident

Chimpanzees, humans’ closest living relatives, do not have chins. Neither Neanderthals, Denisovans, nor any other extinct human species had chins. Humans, it seems, are the only species with the distinct ability to “get punched in the jaw.”

The North American writer Dashiell Hammett mentioned the protruding chin of his detective Sam Spade in the opening sentence of his novel “The Maltese Falcon”.

Spade’s protruding chin was among the facial features which Hammett used to describe his fictional detective’s appearance, but starting with that distinctive chin was, at least from an evolutionary perspective, an involuntary redundancysince all chins are distinctive in the sense that humans are the only ones primates to possess this physical characteristic.

Chimpanzees, the closest living relatives of humans, they don’t have a chin. Neither the Neanderthals, the Denisovans, nor any other human species extinct they had.

Humans, it seems, are like that single species with the distinctive ability to “get punched in the jaw“, because we are the only ones to possess this physical feature. This unique nature makes the chin particularly suitable for identify the A wise man in the fossil record.

In simple terms, a chin is a bony projection of the lower jaw. So why is it there? How and why did it evolve?

The answer, according to one published in January in the magazine PLOS One by a team led by biological anthropologist Noreen von Cramon-Taubadelfrom the University at Buffalo, expands the holistic understanding of the human body as a amalgam of adaptations and random byproducts of evolution.

“The chin evolved largely by accident and not through direct selection, but as an evolutionary by-product resulting from direct selection in other parts of the skull”, states von Cramon-Taubadel, cited by .

The chin is a “spandrel” (a “joint”), a characteristic that appears as a unintended byproduct of evolutionjust as the space under a staircase exists not for any architectural purpose, but as a byproduct of constructing a convenient way to get from one level to another.

In fact, the term “spandrel”, introduced by the famous paleontologist and evolutionary biologist Stephen Jay Gouldwas inspired by the triangular spaces created by the construction of the arches that support the dome of St. Mark’s Cathedral. The spaces have no architectural purpose; they are a byproduct of arches.

The same applies to the chin.

“The fact that we have a unique feature, like our chin, doesn’t mean that has been shaped by natural selection to increase an animal’s ability to survive, e.g. as a buttress for the lower jaw to help dissipate the forces of chewing,” says von Cramon-Taubadel. “The chin is probably a byproduct, not an adaptation.

“It is only by studying the whole that we can understandr rather than aspects of an animal have a functional purpose and what are the secondary products this purpose”, he adds.

Von Cramon-Taubadel and his research team are not the first to suggest that the chin is a “joint,” but their study differs from previous investigations which largely assume that natural selection is the evolutionary driver of changes in the lower jaw.

Instead, they tested the “null hypothesis” of neutrality by comparing cranial traits of monkeys and humans to determine whether evolution was random when it comes to the chin.

“Although we found some evidence of direct selection in parts of the human skull, we found that traits specific to the chin region best fit the spandrel model,” says von Cramon-Taubadel.

“As changes since our last common ancestor with the chimpanzee are not due to natural selection on the chin itselfbut to the selection of other parts of the jaw and skull”, notes the researcher.

Within anthropology, there is a adaptationist tendency in the way people view physical characteristics.

Differences observed between species may contribute to the assumption that all the characteristics were deliberately shaped over timewhich suggests purpose or function, according to von Cramon-Taubadel.

“Generate empirical evidence against this line of reasoning is an important objective of this study and of biological anthropology in general”, he states. “The conclusions highlight the importance of evaluating the evolution of physical characteristics with the integration of traits in mind”.