Why are humans the only species with a chin?

An image of the bottom half of a man's face, showing a dark moustache over a white-toothed smile. The man has an earring bar in his left ear and wears a white shirt.

Humans are the only species with a chin — a feature absent from even our closest relatives. Indeed, it's such a unique anatomical quirk that it's one of the main traits anthropologists use to identify Homo sapiens remains in the fossil record.

Yet, for such a defining feature, we know surprisingly little about its evolutionary purpose. So why are we the only species with a chin?