Although scientists know that only humans have chins, they haven't yet figured out why. There are a few top theories: some have to do with chewing, some with mating, and some with the shrinking of the human face. The chewing hypothesis doesn't stand up to too much scrutiny, as chins don't actually seem to help us chew. That chins could be the result of sexual selection also doesn't make a lot of sense, because both sexes have them; all other mammals limit their sexually selected traits to a single sex. Perhaps chins emerged to reinforce our jaw strength as the human face shrunk over time. But scientists also say that they could be an arbitrary feature, one that just appeared somewhere on our timeline and stuck around.