BUFFALO — Dashiell Hammett mentioned Sam Spade’s jutting chin in the opening sentence of his novel, “The Maltese Falcon.” Spade’s chin was among the facial features Hammett used to describe his fictional detective’s appearance, but starting with that distinctive chin was − at least from an evolutionary perspective − an unintentional redundancy, since every chin is distinctive in the sense that humans are the only primates to possess that physical characteristic.
Chimpanzees, humans’ closest living relatives, do not have a chin. Neither did Neanderthals, Denisovans, or any other extinct human species. Humans, it turns out, have a unique capacity to “take it on the chin” because we’re uniquely in possession of that physical feature. That exclusive nature makes the chin well suited for identifying Homo sapiens in the fossil record.
In simplest terms, a chin is a bony projection of the lower jaw. So why is it there? How and why did it evolve?
The answer, part of a study published in the journal PLOS One by a team led by a University at Buffalo biological anthropologist, broadens the holistic understanding of the human body as an amalgamation of adaptations and random byproducts of evolution.
“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,” says Noreen von Cramon-Taubadel, PhD, professor and chair of the UB Department of Anthropology in the College of Arts and Sciences.
The chin is a spandrel, a feature that arises as an unintended byproduct of evolution, just as the space under a staircase exists not for any architectural purpose, but as a byproduct of building a convenient way to get from one level to the next.
In fact, the term spandrel, introduced by Stephen Jay Gould, was inspired by the triangular spaces created by building the arches supporting the dome of the San Marco Cathedral. The spaces have no architectural purpose; they are a byproduct of the arches.
The same is true of the chin.
“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,” says von Cramon-Taubadel. “The chin is likely a byproduct, not an adaptation.
“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.”
Von Cramon-Taubadel and her research team are not the first to suggest the chin is a spandrel, but their study differs from previous research that largely assumes that natural selection is the evolutionary driver of changes to the lower jaw. They instead tested the “null hypothesis” of neutrality by comparing cranial traits of apes and humans to determine if evolution was random where the chin is concerned.
“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,” she says. “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.”
Within anthropology, there is an adaptationist bent in how people view physical characteristics.
Observed differences between species can contribute to the assumption that all characteristics have been deliberately shaped over time, which suggests purpose or function, according to von Cramon-Taubadel.
“Generating empirical evidence against that line of reasoning is an important goal of this study and biological anthropology in general,” she says. “The findings underscore the importance of assessing the evolution of physical characteristics with trait integration in mind.”