點解男人有乳線?

When a sperm and an egg fuses together to become a foetus or embryo, whatever you wanna call it, it is not yet decided if it is a female or a male until about 3 months after the formation of the embryo. The template which is also the embryo will have all the characteristics of both male and female including nipples, apart from their sexual organs which will be developed after the sex is determined.

之前聽我學校的呀sir講....
因為人類係媽媽的肚的時候已經形成左一個初形..
then先分別媽媽同爸爸的染色體...
來分別bb的性別...~
so一開始個bb已經係有乳線......

無用的器官會自然消失。
但人類進化也需一大過程。