Why do we have a chin?

THAT'S THE WHY:When our kids were younger we had a CD of songs from Sesame Street playing on continuous loop in the car. And sometimes during the quiet moments, stuck in traffic, it struck me how curiously deep some of those lyrics could be.

In the classic One Fine Face, Ernie (backed up by a willing Elmo) goes through the anatomical features of the human visage. All highly educational fun of course, and this catchy little couplet in particular caught my ear: “I’ve got a nose so I can smell. I’ve got a chin, but what a chin does I just can’t tell.”

The last line hits on a question that has long puzzled researchers: why do we have chins?

No doubt Ernie is well up on the literature, but for those of us who aren’t, several theories have been put forward.

Maybe the human chin developed its shape as our mouths and teeth reduced in size. Perhaps the chin we know and love today is a good fit to absorb the various mechanical stresses generated by speech.

But it could also be that we like them that way: a paper last year in the American Journal of Physical Anthropology argued that chin shape could be influenced by sexual selection. It’s still an area that’s open to debate, so Ernie might be scratching his mysterious smig on it for a while yet.