Why do men have nipples?

In the “I didn’t know that” category, a New York doctor, who authored the book, Why Do Men Have Nipples? offers this explanation:

While only females have mammary glands, we all start out in a similar way in the embryo, the authors explain. The embryo follows a female template until about six weeks, when the male sex chromosome kicks in. Men, however, have already developed nipples.

Makes me wonder if Adam really did come first. But I’m not supposed to question that.

Via Yahoo! News