AsapSCIENCE brings us an important lesson on human developmental biology. All men - yes, even the manliest of men *cough* - began as females. Sort of.
See, the default developmental pathway for a zygote is toward female. That’s due to signals that come from genes on the X chromosome. In XY males, however, a special Y-chromosome gene inhibits just a few of those X chromosome genes and tunes the embryo to respond to testosterone. This takes what would become female parts and “adjusts” them slightly to become the second male brain.
Yep … your family jewels used to belong to the princess. This also answers the “why men have nipples" question.
On a weird science side note, when this system malfunctions in development, the results can be pretty interesting. In a condition called androgen insensitivity syndrome, the embryo is XY, but can’t respond to those early testosterone signals. Instead, it follows the “default” pathway and develops all of the outer female anatomy (vagina, etc.) but none of the inner (ovaries, uterus). Male, but female!
Bilateral gynandromorphism is a rare genetic disorder occurring in insects, arachnids, crustaceans, and birds, where a strange combination of genetic material splits a creature perfectly in half, with one side male and one side female.