Chin Up!
- marketingc8
- 20 hours ago
- 3 min read
Word of the day: spandrel.
It’s most commonly used in architecture. It’s the triangular space between the curve of an arch and its rectangular frame.
But in biology, the same word describes evolutionary traits that are byproducts of other developments.
Examples of spandrels in the human form include:
Human facial structure. The overall shape of our faces (including reduction of the brow ridge) could be the result of brain expansion and smaller chewing muscles, among other changes.
The belly button. This is a necessary but non-functional byproduct of the placental attachment during development.
Male nipples. These are a developmental consequence of a shared, neutral embryological plan in mammals, serving no physiological purpose in males.
The human chin.
Yes, the human chin. In fact, this is considered the classic spandrel, although there was debate for decades whether the chin qualified or not.
The chin is unique to Homo sapiens. The chin has no clear functional purpose and likely arose from the reduction of the jaw and changes in facial growth patterns.
Even chimpanzees, our closest living relatives, do not have a chin. Neanderthals? Nope. Denisovans? Nope.
Note, the chin is not the mandible. The mandible, of course, is the entire lower jawbone. The mandible holds the bottom teeth and enables chewing. The chin is only the forward-projecting, thickened bony prominence at the very front-center of the mandible.
So why did we need a chin?
Well, a study published in PLOS One (PLOS is the Public Library of Science) and conducted 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.
According to Noreen von Cramon-Taubadel, Ph.D., professor and chair of the UB Department of Anthropology in the College of Arts and Sciences, “the chin evolved largely by accident and not through direct selection.” The chin, she said, is an “evolutionary byproduct” of our development.
"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.”
As this article on DiscoverMagazine.com stated, the researchers compared 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.
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 said in an article on Phys.org. "The findings underscore the importance of assessing the evolution of physical characteristics with trait integration in mind.”
Evolution is not the only force in how we’re shaped. Some traits are accidents.
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