I was gonna say elephants have chins too, as I'm re-watching old QI seasons and they brought up this fact, but upon further research their chins aren't true chins with a bony protrusion.
“If you're looking across all of the hominids, which is the family tree after the split with chimpanzees, there [are] not really that many traits that we can point to that we can say are exclusively human,” Duke University’s James Pampush tells Robert Siegel for NPR. “[T]hose animals all walked on two legs. The one thing that really sticks out is the chin.”
One of the most popular ideas is that our ancestors evolved chins to strengthen our lower jaws to withstand the stresses of chewing. But according to Pampush, the chin is in the wrong place to reinforce the jaw. As for helping us speak, he doubts that the tongue generates enough force to make this necessary. A third idea is that the chin could help people choose mates, but sexually selective features like this typically only develop in one gender, Pampush tells Siegel.
The spandrel hypothesis is as good a theory as any, but it too has its problems. It’s hard to find evidence to test if something is an evolutionary byproduct, especially if it doesn’t serve an obvious function. But if researchers one day do manage to figure out where the chin came from, it could put together another piece of the puzzle of what makes us different from our primate and Neanderthal cousins, Yong writes.
The "spandrel hypothesis" is the front runner explanation. Essentially we didn't evolve to have chins but rather evolved other things that are helpful, and the chin is a byproduct of that other evolution. Not harmful so it didn't get selected away, but not helpful.
I read somewhere sometime ago that the theory that makes the most sense is that we evolved chins to take a punch, which animals besides our immediate evolutionary relatives do not do.
So we evolved chins as an evolutionary advantage over our immediate evolutionary relatives who would logically be competing for the same resources.
The chin is situated near the area where the tongue and jawbone interact during speech. It's possible that the chin provides a surface for the tongue to move against, allowing for more complex sounds and articulations. The development of language is believed to have occurred around the same time as the emergence of Homo sapiens. While other primates have similar facial structures, they don't possess a distinct chin. This suggests that the chin might be related to the unique demands of human language.
Bingo -- other animals don't have a chin because they didn't invent languages like humans did for communication, and thus the demands of speaking weren't evenly distributed.
Next time on interesting questions 104: Why did homo sapiens develop language when other animals such as Corvids did not?