After conception the developing embryo follows a female template. It is only after 6-8 weeks that the effect of a gene on the Y chromosome kicks in for boy embryos. This gene stimulates the production and influence of hormones such as testosterone which 'masculinise' the embryo by altering physical development to form male features. But nipples are formed very early in the embryo, before this masculinisation process takes place, and nothing that goes on later in fetal development reverses nipple formation. So the story that God made Eve from Adam's rib got it the wrong way round. Males are an adaptation of females and the nipples are just one piece of evidence of this. Evolution may hold an answer. (Source BBC) That last sentence is interesting.