Humans have chins. That shouldn't be particularly surprising. What might be more interesting ( unless you've been keeping up with EvoAnth) is how unique our chins are. No other ape has a chin as prominent as ours (and no close member of our family does either). This is rather peculiar. After all, it doesn't seem to contribute to our survival; so why did it evolve?
There is the possibility that it didn't evolve. Or rather, the chin wasn't specifically favoured by natural selection. Evolution includes a degree of randomness, causing some traits to become common through sheer chance. Could the chin be one of them; hence why it seems to offer no survival advantage?
This seems like a nice, easy explanation for a rather peculiar feature. Unfortunately, it doesn't seem to be true. A scientist examined the chins of other apes and found that ours is so unique and developed so quickly it could only be explained by natural selection. Our chin evolved because it conferred some benefit to our survival.
Now; to be fair this doesn't completely remove the possibility of chance. Sometimes genes located close to one another on the genome can be associated; so selection for one leads to selection for the other. If the chin became associated with big brains that could explain why it developed. Yet such an explanation couldn't account for our weird chins arising in the first place. Perhaps something else is afoot (or achin).
Unfortunately this research doesn't really help resolve this issue. The list of possible alternatives is long and there's not much we can (at this moment) to separate them. Perhaps it was a product of sexual selection, maybe it was a bi-product of other adaptations in the jaw, or it could be a change to deal with stresses during chewing. The list of such possibilities goes on.
But importantly, we now know that this is a list that needs examining. Our weird chin was not an accident, but developed for a reason. It took us 100 years of studying to identify what that reason was; it could take a hundred more to find out why.
Hopefully it won't, but it could.
Reference
Pampush, J. D. (2015). Selection played a role in the evolution of the human chin. Journal of human evolution.
