In humans, we all start out as female in the womb. Hence why male-bodied individuals have nipples.
Additionally, ovaries and testicles aren’t all that different. Both begin their lives as the same gonads in the womb.
To oversimplify the fuck out of microbiology, the SRY gene attached to the Y chromosome tells the gonads to descend and form into testicles. The absence of the SRY gene results in the gonads developing into ovaries.
Apparently it’s possible for developed ovaries to switch to testicles too. A 2009 study conducted by geneticists at the European Molecular Biology Laboratory in Heidelberg, Germany did an interesting experiment with mice. By removing the FOXL2 gene in mature mouse ovaries, the ovaries ended up producing testosterone after just 3 weeks.
Penis Vs. Vagina
Our genitals also aren’t terribly different from each other. It’s primarily the introduction of dihydrotestosterone (DHT) that turns what would be a vagina into a penis.
By the sixth week of pregnancy, the genital area of the fetus develops into a genital tubercle. This tubercle is an arrangement of tissue in the genital area that is the same for both males and females until around week eleven.
The introduction of DHT molds the tubercle into the glans penis, where the absence of DHT results in the formation of the glans clitoridis.
Prior to all this, the urogenital area is just an open cavity for both males and females. This cavity ends up getting fused together, which results in a scar called a raphe line that runs from the anus to the perineum in both males and females. In biological males, however, this raphe line extends up the midline of the scrotum and sometimes continues partway up the penis.
This raphe line, then, is a cruel reminder of what I could have been. I’m going to name it Ralph. Fuck you, Ralph.
I’ve made formal complaints to this god person. I told them “I think my urogenital cavity closed up on accident during the manufacturing process, do you think I could return my penis and be refunded a gift card good for the amount of a replacement vagina?” but they said since it’s already used they can’t accept a return. Go figure: not even a year into being a woman and already I’m being slut shamed.
The Müllerian duct is another interesting fetal structure. This duct molds into what eventually becomes the Fallopian tubes, which eventually branch off into the uterus, and they exist in both males and females. It’s the introduction of Müllerian Inhibiting Substance (MIS) in males that causes the regress of the would-be Fallopian tubes.
Women also produce MIS, but where males produce more in infancy and gradually produce less and less until puberty, females often have no detectable traces of MIS in infancy and gain progressively more as they age.
What’s Up With Social Norms Then?
So you see, males and females aren’t terribly different from each other. Many of our differences are the result of one or two minor gene or hormone differences.
For instance, if a person with XX chromosomes has their FOXL2 gene inhibited, then their gonads will end up developing as testicles. If a person with XY chromosomes doesn’t produce enough MIS, then they’ll develop Fallopian tubes and eventually a uterus (which does happen). If a person born XY doesn’t get enough DHT, then they develop a vagina (or perhaps ambiguous genitalia).
This makes it unsurprising that intersex people make up around 2% of the population, or the same number of people as their are redheads.
It’s fascinating how such minor deviations result in such firm social norms though.