The Atlantic says no one knows why. Apparently, it’s not for chewing, finding mates, speaking, or taking punches.
[Anthropologist James] Pampush doubts that chins are adaptations at all. He thinks it’s more likely that they are spandrels—incidental features that have no benefits in themselves, but are byproducts of evolution acting upon something else.
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A different explanation portrays the chin as a bit of the jaw that got left behind while the rest shrunk back. As early humans started cooking and processing our food, we made fewer demands upon our teeth, which started shrinking as a result. They gradually retracted into the face, while the part of the lower jaw that held them did not (or, at least, did so more slowly). Hence: chin.
Stephen Jay Gould and Richard Lewontin, who coined the concept of evolutionary spandrels, liked this hypothesis. So does Nathan Holton from the University of Iowa, who studies facial evolution. “It seems that the appearance of the chin itself is probably related to patterns of facial reduction in humans during the Pleistocene,” he says. “In this sense, understanding why faces became smaller is important to explaining why we have chins.”
“But why did the lower border of the jaw also not shrink?” Pampush asks. “What happened that left that last little bit sticking out?” This is the problem with spandrel hypotheses more generally: They’re often very hard to test. More.
Yes, we were just thinking that. Hard to test… Interestingly, Pampush notes, “The chin is one of these rare phenomena in evolutionary biology that really exposes the deep philosophical differences between researchers in the field.” Say more.
That’s overdue for discussion, it seems.
See also: Why do we need less sleep than chimps?
Rethinking evolution mechanisms
and
Human origins: The war of trivial explanations
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