Evolution of mankind from lower forms to lesser developed primates to present day Homosepians has been a process of slow and functional metamorphosis. Evolving to the present day form, though human race kept shedding unused body organs with time, yet the presence of a few, which have continued to exist despite being of least practical use since ages, is inexplicable and intriguing. Our ancestral primates had a tail which vanished in due course of time. The tail’s disappearance was logical as when we deboarded the trees and started to stand on two legs, it became a non performing asset and thus lost with time.
But few things like the vermiform appendix and the male human nipples, we did not lose despite being of no use. The female nipple and the whole of milk producing mechanism is anatomically similar to the one in males to the last detail but in males it is functionally defunct since prehistoric times. The lactating glands, also called mammary glands, are basically modified sweat glands with nipples as a common oozing source for many glands. The milk producing glands (Lobules) , the ducts which carry milk from lobules to nipples, the fatty and the connective tissue surrounding the ducts and the nipples, are all present in both the males and females of the species.
From the embryonic stage to fetus to birth and till puberty there is no difference in the anatomical development of this intricate system in both men and women. The disparity sets in only after the release of female hormones at puberty, when in females the breast ducts grow and lobules are formed at the end of these ducts. The amount of tissue (stroma) surrounding the ducts and lobules also multiplies considerably. But despite all the augmentation, functionally the mammary glands in females get activated only after pregnancy. This female hormone, which brings about such noteworthy changes in female anatomy with the onset of puberty, is negligibly present in males too. But it’s trivial level brings insignificant changes in male anatomy. The ducts remain small, few or no globules get formed and there is a marginal increase, if any, in the tissue around nipples.
Finally, the answer lies in the embryonic stage itself. It is now a widely researched, published and accepted fact that in the early stages of development, the embryo is genetically female and is independent of hormonal influences. It is only after four to six weeks of embryonic stage that the prenatal development of the breast tissue comes under the influence of gender specifying characteristics. It is only after that much time into development of fetus that male chromosomal traits and hormones start displaying their characteristics. In a step by step development of the embryo, the gender determining factors start playing their role after the predominantly female embryo has undergone the breast and the nipple formation stages. Once these organs are formed, there is no reversal or degeneration in males, while in females, they start contemplating further changes at puberty, and eventually getting activated after pregnancy.
Comprehensively, it may be understood like this; that the human body comes fitted with a gadget to be used by the one who can complement it with necessary hormones. In this case since only female hormones can do the job, the male nipples and the mammary glands remain dormant and least to say, ornamental.
It has also been contended that the ever evolving human race, is in a transition phase of diminishing milk producing mechanism in men and that with the passage of time, male nipples as also the entire lactating system will be gone for good.