The embryo starts with a shared body plan
In early development, an embryo does not begin as a miniature version of either a typically male or typically female body. Many structures follow a common plan before hormonal signals guide them along different paths. Nipples are among the features laid down during that shared phase.
The early skin forms paired strips of thickened tissue called milk lines, which run from the armpit area toward the groin. In humans, most of each strip fades, while a small region on each side of the chest develops into a nipple and the tissue beneath it.
What changes when male development begins
A working SRY gene, usually carried on the Y chromosome, can prompt an early gonad to develop into testes. The testes then produce hormones that guide the formation of many typically male reproductive traits. Those signals arrive after the early body plan—and nipple structures—are already underway.
The hormones do not normally remove the nipples. They do, however, affect how breast tissue develops: in people with typically male development, it generally remains much less developed, while later hormonal changes can stimulate breast growth in people with typically female development.
A shared feature, different later development
Nipples are therefore not a separate adaptation added only to one sex; they persist in both because they form before development diverges in many respects. Their presence also shows that a feature can be established early even when its later growth or function differs.
That shared starting point is visible on the outside: the same basic pair of nipples can remain while the surrounding reproductive anatomy follows different developmental paths.
