We all have nipples. Ladies need them to breastfeed infants, but why do guys have them? Not to sound trans or anything, but way back in our most early development days, we were actually girls. Sort of.

You know, our modern human bodies have a number of “Not-so-functional” features like wisdom teeth and tail bones, but male nipples are different. They aren’t left over from some evolutionary event in the past when males used to breastfeed. They’re artifacts of our own personal development. In the womb, mammals go through a series of developmental stages as they grow from a bunch or cells to a screaming infant. The process goes zygote, embryo, fetus, baby. Most babies are usually male or female, girls with two X chromosomes and boys with an X and a Y.

But as embryos, we start out following a female blueprint before our hormones get serious about gender, so the nipples actually develop before our gender is determined. Within the first several weeks, a pair of milk ridges form on every embryo, a thickening in the epidermis that runs from armpit to thigh. Eventually, these structures pull back to form two nipples, but in some cases, (like that of Chandler Bing from the Friends TV series) we sometimes develop a third nipple, which some people sweep under the carpet by claiming it is just a mole..

Shortly after nipples form, in babies with a Y chromosome, a special gender-determining gene switches on and declares the embryo officially male, kickstarting the development of male hormones and eventually male anatomy. But we get to keep the nipples.

Okay, but still, if they’re so useless, why do we have them? Well, the separation of male and female traits in a species, or uncoupling, tends to happen only if there’s a good reason for it, in terms of reproductive success. Evolutionarily speaking, female breasts are essential to the survival of a species. Male nipples aren’t, but why mess with something that’s so important to one sex and merely benign to another? So dudes don’t have nipples because those features have been selected for in males, but rather because simply they haven’t been selected against. Not all traits have adaptive explanations. In the end, male nipples are just a genetic byproduct of female breasts.