Small, nonfunctional third nipples are not unusual in humans. These kinds of evolutionary leftovers, like the rare occurrence of a human tail, are what are known as vestigial traits, or atavisms. As the manifestation of an organism's evolutionary history, vestigial traits are stamps in the passport of evolution. Some mammals, like mice and dogs, have multiple mammary glands for feeding litters of young. Humans, like most primates (but there are some exceptional strepsirhines with multiple nipples), normally give birth to one baby at a time that requires only a pair of mammary glands for nursing. Extra nipples on humans symbolize the common ancestry with other mammals that have more than two teats.
Nipples on male mammals are not vestigial traits, as they do not signal an ancient state of male milk production. Male nipples are simply the result of an interesting developmental process. All humans begin development as a female and not until about the seventh week does an embryo with a Y chromosome start developing as a male. This transition occurs after the development of the skin on the chest, but before the development of the mammary glands underneath it. Male nipples are simply stamps in the mammalian passport of development.
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