Ed Yong has an article in Atlantic on the mysterious evolution of the human chin: “We’re the Only Animals With Chins, and No One Knows Why”. He builds on a recent review paper by James Pampush.
It’s a nice read about a trait that has occupied more than the usual amount of paleoanthropological mindshare over the last hundred years.
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.”
There are some true “chin nerds” out there who will likely think the article doesn’t treat their favorite hypothesis fairly. The fact is, it’s sort of embarrassing that we have not yet come up with a persuasive test of any of these hypotheses.