Ever wondered why male Homo sapiens have nipples on their chest? Are they vestigial like the appendix, the wisdom teeth or body hair? Or they’re just present for weird piercings or to taunt hungry infants?
Well, the answer is quite interesting. Turns out, human males have nipples because at some point of the embryonic stage, we were all girls. Yeah, you heard me tough guy!
Unlike the other vestigial organs, nipples aren’t those organs that are just left overs from any evolutionary event where male humans used to breast feed, at any point in the course of evolution male humans did not have mammary glands to do so. So the evidential fact that male humans have nipples is kind of flattering. With that in mind, male mice are the only mammals that do not have nipples, so one might think they’re probably tougher than us guys! Thus the question still remains, why do males have them?
The human gestation period goes from zygote, to embryo, to foetus and then to a baby, which is either a male or a female. As the sex of a baby is determined whether one chromosome from the dad is either an X or a Y, what’s interesting to know that all human embryo’s start off with a female blueprint, i.e. presuming to have two X chromosomes. (Yes, we were all girls at one point of our life). Up until the sex is determined (activation of the Y chromosome), the process of a typical female embryo development is already started. Within the first several weeks, pair of ‘milk ridges’ or milk lines form on every embryo, either male or a female, which serves as the foundational mammary tissue for the development of nipples and mammary organs. After the sex is determined, i.e. one of the sex chromosomes becomes a Y, a protein called PHTrP is synthesised which causes the embryo to develop male hormone receptors.