The simplest organisms reproduce by copying their OWN genetic code- the next step was sexual reproduction; creating a new set of genes out of two old ones. So what caused some organisms to lose their ability to choose any partner in their population, having instead to choose a partner of the opposite sex? Perhaps a mutation of an X chromosome in a single-celled creature into a Y and the eventual change of the hermaphrodite X chromosomes into female Xs? Just a guess... Does anyone know? Additionally, does the fact that the males of many species have nipples have anything to do with this? If not, why are they there?