The Embryo's First Few Weeks
During an embryo's first few weeks a precise blueprint for development is followed. This blueprint is the same for embryos that are male and female; in fact, during these first few weeks you can't tell a male and female embryo apart. It's during these early weeks that the nipples form on all embryos. It's not until later that changes occur. It's during these changes that the embryos become male and female.
Things for the embryo begins changing during week six. This is when testes are formed, and in later weeks the male embryo begins to make testosterone. During this time changes in the genitals and brain cells begin. By the time these changes occur the nipples are already in place for both male and female embryos.
Mammary tissue growth is suppressed by the testosterone. If male embryos didn't begin to produce testosterone during week nine of development, breast development would occur later in life. However, not all mammary tissue growth is suppressed. Many males do experience tissue swelling during puberty. This swelling is short-lived and disappears in a year or two, and is always gone by the time a male reaches 20 years of age.
It might stand to reason that since nipples serve no purpose for men they would have disappeared by now. After all, many useless body parts have disappeared, thanks to evolution. Yet, men still have nipples.
One reason a man's nipples remain is because they don't cause any problems for the man. The fact that nipples aren't detrimental is what has likely kept them around. If having nipples were harmful to the man, they would have disappeared by now. Nipples just weren't an evolutionary priority.
Vestigial structures are unneeded body parts. Men's nipples are not the only vestigial structures that humans have. Others include the appendix, wisdom teeth, body hair, the tailbone and arrector pili. Arrector pili are the small muscles that are attached to hair follicles. These muscles are what cause goose bumps. All of the above are things that humans no longer have a use for, yet they still remain.
No one knows whether or not evolution will one day do away with nipples in men. There are two schools of thought on this topic. There are those who feel that humans have evolved as far as they can and those who think humans can further evolve. Those who believe that evolution will continue believe that one day male nipples will be a thing of the past, along with wisdom teeth, the appendix and most body hair. Those who believe humans are done evolving feel there will be no more changes and everything humans currently have, whether it's something that's needed or not, will remain with us. Only time will tell which side is correct. But at least for now male nipples are staying.