Something that has always stuck with me since Intro to Biology years ago was something my professor said: females are the prototypical human sex. In other words, we all start out as females in a sense. It is not until around the 8 week benchmark in fetal development that a fetus starts to develop distinct sex characteristics. Generally speaking, from the moment of fertilization the embryo has an innate genetic sex (XX in a female; XY in a male). However, if you were to see a fetus before the 8 week mark, you would see that the genitalia is undifferentiated between XX and XY. That means that a female fetus and a male fetus look the same between the legs, so to speak.
So, why is that? Why do female and male genitalia not differentiate from the moment of fertilization? And what causes the differentiation when it happens? And what did that professor mean by saying that females are the prototypical humans? Well, it all has to do with the Müllerian ducts and hormonal reactions during pregnancy.
The Müllerian ducts are a set canals in the urogenital region of an embryo (that is, where the urinary and genital structures develop). Depending on which way sex differentiation goes, the Müllerian ducts develop into the Fallopian tubes, uterus, and upper part of the vagina or they will begin to disappear leaving only small vestigial remains. (The male reproductive organs develop out of the adjacent Wolffian duct. These ducts begin to disappear during sex differentiation in females.) Here is a diagram:
For reference, here is a list of homologous human reproductive structures (for instance, before sex differentiation, the scrotum and the labia majora are one and the same).
It is around this 8 week period that hormones are released from within a male fetus from the testes (from cells called Sertoli cells). This is called the anti-Müllerian hormone. The chromosomes of a male fetus receive this hormone and react by impeding the development of the Müllerian ducts. In a female fetus, the chromosomes do not exist so the Müllerian ducts continue to develop. From time to time, the necessary chromosome to inhibit the development of the Müllerian ducts in the male are missing. (Remember that human chromosomes are incredibly complex, so when DNA is being “written” sometimes it can make a “typo.”) When this happens, the fetus continues to develop the Müllerian ducts. So, the genetically male fetus begins to grow a uterus and sometimes other female reproductive structures. Usually, the testicles do not descend but a penis will still be present because it does not develop from the Müllerian ducts. This is one of the many complications of determining sex at birth. It may not be immediately clear what the child’s sex is. This is called Persistent Müllerian duct syndrome (PMDS) and can also result as a failure of the testes to ever secrete the hormone.
So, going back to the words of my professor, all humans begin as embryos with the same “feminine” appearing genitalia. This is why he says, with glee, that females are the prototypical humans. Keep in mind all that the ancient Greek philosophers argued (and Freud, for that matter) that men were the essential human form and that women are defective versions of males. Turns out, that all men start out as women in a manner. There is a lot more to be said on this topic, but I will stop here for now. Use the information you have learned here to impress your friends and put any obnoxious men in their place. Please feel free to leave comments and questions.