Males and females do, in fact, start from the same genetic blueprint. Embryos, in their first weeks, develop structures with the potential to become either male or female sex organs. Only later do sex-specific hormones begin to mold those structures into either a clitoris or a penis, either functioning mammary glands or unproductive male nipples. “Males and females are not separate entities, shaped independently by natural selection,” [evolutionary biologist Stephen Jay] Gould writes. “Both sexes are variants upon a single ground plan, elaborated in later embryology.” Nipples begin to appear before that sex differentiation begins, so males are stuck with them as they mature.
It’s not entirely fair to say they serve no possible purpose, though. Under unusual conditions, like starvation or a spike in levels of the hormone prolactin, men can actually produce milk.
Nipples are also highly sensitive, and research shows that in men as well as women they respond to sexual stimulation. One study found that about half of men (and more than 80 percent of women) report nipple stimulation enhances their sexual arousal. Maybe these minor compensations, rather than a boost in physical and reproductive fitness, justify the male nipple’s existence.