Have you ever wondered how you ended up with your penis or clitoris? Not in a philosophical or historical sense—like, wondered where it literally came from and what it's made out of?
Wonder no more: For the first time, scientists have figured out exactly how the penis and clitoris develop in the embryonic stage, which cells they initially come from, and why we don't have two sets of each.
As many as one in 250 people are born with some sort of (usually correctable) genital defect, most commonly a condition called hypospadias, in which the urethra is incompletely closed. The scientific reason for that, as far as these researchers can tell, is that when we're developing, we have two half penises or clitorises that eventually fuse into one.
The fact that we start off with two of whatever's down there may sound like a bit of a mindfuck, but from what we know about evolutionary biology, it's not all that surprising.
Animals are rife with homologous structures—the structure of the wing of a bat is similar to a whale's flipper which is similar to our hand. When we're developing in the womb, those similarities are even more pronounced, as we often look like pigs or chickens or other animals as we're going through our whole embryo thing.
Knowing that, it's not crazy to think that, in the womb, we have two genital structures for some time, considering that adult snakes and lizards have two genitalia (called hemipenises and hemiclitores, respectively).
While not exactly homology in the strictest sense (researchers call it "deep homology"), it's the same sort of theory: The genes that code our genitalia are believed to be similar to the ones that code snakes'.
"The phallus has a composite origin, in which the left and right sides are lineage-restricted compartments of progenitor cells that are brought together during body wall closure," Martin Cohn, a researcher at the Howard Hughes Medical Institute and author of the Scientific Reports paper, wrote. "A common mechanism involving disruption of posterior body wall closure could underlie both [hypospadias], and this might explain their frequent association in humans."
So, why two penises in snakes and lizards and one in mammals and birds?
Cliff Tabin of Harvard Medical School, one of the authors of the accompanying Nature paper, told me in an email that the penises and clitorises of amniotes (mammals and birds) and squamates (snakes and lizards) are "created by the same genetic program."
Despite that, our genitalia are made out of slightly different cells than our squamate brothers and sisters. His team and Cohn's dyed certain cells in developing animals and discovered that the genitalia of squamates are actually made out of the same material that makes hind limbs in lizards, whereas ours come from the tail bud.
"When we fate mapped the embryonic origins of these structures we found that the genital tubercle (primodium of the penis and clitoris) derives from the tail bud," Tabin wrote. Meanwhile, "the hemipenis derives from the lateral plate mesoderm, the same tissue as makes the hind limbs."
With that knowledge, Cohn notes that snake penises can considered to be "true half phalluses," because ours start as two but turn into one, whereas snakes' start as two but remain separate.
At the moment, there isn't a whole lot we can do clinically with this information, but you can be thankful that you don't have two half penises or clitorises, I suppose.