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.
For example, during human evolution, our faces shortened and our posture straightened. These changes made our mouths more cramped. To give our tongues and soft tissues more room, and to avoid constricting our airways, the lower jaw developed a forward slope, of which the chin was a side effect. The problem with this idea is that the chin's outer face doesn't follow the contours of its inner face, and has an exceptionally thick knob of bone. None of that screams “space-saving measure.”
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.
It may seem frustrating to have so many imperfect competing hypotheses, but that's part of the joy of chins: They reveal something about how scientists think about evolution. Some see the sculpting power of natural selection in everything, and view chins as surely some kind of adaptation. Others see natural selection as just one of many evolutionary forces, and so gravitate towards a spandrel-based explanation. “The chin is one of these rare phenomena in evolutionary biology that really exposes the deep philosophical differences between researchers in the field,” says Pampush.
And, indeed, between people outside the field. “I always get entertaining emails from lay people trying to help me so let me thank you in advance for what I'm about to receive,” he tells me.
Because if there's one trait that more universally human than the chin, it's having opinions.