Two facts about chins: (1) Only humans have them. (2) No one knows why we have them.
The chin isn’t just specific to humans: it’s specific to modern humans. Earlier forms of us didn’t have them. I think it shows up around the time of Cro-Magnon Man, the earliest form of modern humans, about 45,000 years ago.
Homo erectus was around from about 1.9 million years ago until about 70,000 years ago. It’s probably an ancestral species to modern humans. No chin, though.
Neanderthals were around from maybe 250,000 years ago until about 40,000 years ago. I’m not clear on the arguments as to whether or not they’re ancestral to modern humans, but we probably inbred with them. No chin, in any case. (Note: I’m not a big fan of arguments that are only backed up by a single data point. For lots more pictures of Neanderthal skulls, go to Google Images. You still won’t find any chins.)
I’ve read that human infants don’t have chins, but rather they develop over the course of growth. From the skulls that I’ve looked at, this isn’t true–if you look at a human infant’s skull and the skull of any of a variety of apes, the human infant skull looks pretty distinct to me, in part because of the presence of a chin. A tiny, not-very-protuberant chin, sure–but, a chin nonetheless.
Do we really not know why modern humans have chins? We really don’t. Which is to say: a number of proposals have been advanced, but none of them is very convincing. Some of those proposals:
- Chins protect the lower jaw from the mechanical stresses of mastication (chewing).
- Chins protect the lower jaw from the mechanical stresses of speaking.
- Chins are what is left behind after the rest of the face shortens over the course of human evolution. (Look at how far the adult chimp’s face sticks out in the series of drawings of human and chimp skull development; then compare the adult human face, which doesn’t stick out.)
- Chins are meant to deflect blows to the face.
- Chins come from unspecified “changes” related to reduction in testosterone levels over the course of human evolution.
None of these is a great explanation; some of them are very bad explanations; all of them are difficult to test. For some approaches to thinking about these various and sundry proposals, see any of these pages:
Some relevant French vocabulary for talking about the chin:
- le menton: chin.
- mentonnière: Although I can’t find this in the dictionary as an adjective, I think that it can be: Le mouvement volontaire de saillie mentonnière est assurée par l’extrémité de la sous-unité corporéale de l’os mandibulaire et par le muscle releveur du menton (aussi nommé houppe du menton ou incisif inférieur). “The voluntary movement of chin-projecting…” Can a native speaker verify this?
- la mentonnière: chin-piece.
By the way: as MELewis has pointed out, if you want to have this discussion in any sort of detail, it’s important to have a definition of “chin.” In fact, depending on how you define it, you might want to say that elephants have independently involved a chin. Here are some pictures: an elephant skull, a mammoth skull, and a mastodon skull. All three of them show chin-like structures.