Yesterday, I wrote about the mystery of the human chin : We’re the only animals with chins, and no one knows why. In response, several readers argued that other animals, including horses, cats, and elephants, do have chins.

The critical point here is that a chin isn’t just “the bottom bit of the face” or “the front part of the lower jaw.” It’s specifically a lump of bone that protrudes forward from the lower jaw. Do a Google image search for “horse skull” or “cat skull” and you’ll note that no such protrusion exists. The jaw slopes sharply backwards from the teeth: no chin. When you stroke your cat’s “chin,” you’re stroking its mandible. You can call it a chin, but that’s a colloquialism; it doesn’t bear upon the evolutionary question of why humans and humans alone have that forward-jutting bit.

With elephants, things are a bit more complicated. The elephant lower jaw does have a forward-jutting bit that certainly looks pretty chin-like. Then again, their lower jaws are really weird. Elephants have lost the front teeth of their lower jaw for some reason, leaving a vacant space that creates the illusion of a bony promontory. They also have a fleshy lower lip that doesn’t mould to the contours of their front teeth like ours, but instead sticks well forward, tapers to a point, and acts as a mobile appendage to oppose the trunk. So that front bit of the jaw has a clear function: it acts as an anchor and platform for this unusually mobile lip.

“For these reasons, it is generally agreed that whatever the biological situation occurring on the front of the elephant lower jaw, it is fundamentally different from the condition in humans,” says James Pampush, who recently reviewed the various possible origin stories for the human chin. “It may provide some insights into the situation in humans, but to call it a 'chin' stretches the definition.”