It doesn't matter what sex you were born, every single one of us is the proud owner of nipples. While the vast majority of us have but two nipples, in rare cases, a third, supernumerary nipple can form - even on the sole of the foot. It makes sense that us females have nipples, because just like all other mammals, we need them to deliver milk to our offspring. But what about males? Why hasn't natural selection done away with this apparently useless knob of flesh?
As this episode of It's Okay To Be Smart explains, it's not so much that male nipples are pointless, it's the fact that they're relatively cheap for the body to produce, and they're not doing any harm, so quit bothering evolution with your dumb questions and let it get on with the important stuff like... getting rid of our appendixes and wisdom teeth?
Turns out, dropping an entire body part through natural selection is not as simple as "We don't need it, so let's get rid of it." Programming cells to form a couple of fleshy nubs on your chest is one thing, but reprogramming the genes and the body development process of just one sex in a species is going to cost far more.
"Traits don't have to provide a specific advantage to get passed on," says Joe Hanson in the video above.
Consider blood - ours is red, and an octopus's is blue. It's not a case of one colour being 'better' than the other - it's just a case of nature selecting different chemistry for different lifeforms. We've got a lot of iron in our blood, and octopuses have a lot of copper in their blood. "Not every trait is an adaptation, and they don't all have a point," says Joe.