When my wife and I were expecting our child, I remember feeling like things weren’t quite fair. What was the point of this division of labor? Let me clarify that I am very much a straight cis man, comfortable with my body. But I felt a bit insecure about the size of my contribution to the whole endeavor. I wished I could ease her burden by sharing the load. Conversely, I envied the intimate proximity of pregnancy and nursing which no relationship in my life would ever rival. Mammalian reproduction is like hiring two one-armed workers to paint your house with a single brush, so Lefty is just there to help open the can at the start. Such an arrangement works, but it’s not exactly a paragon of efficiency. Why do we do it this way?

Two of the species I work with, bloodfluke-transmitting planorbid snails and woodland strawberries, are hermaphrodites. Hermaphrodites are simply organisms with both male and female function. That is, they can make both eggs and sperm (or pollen). For the snails, this is a research convenience for us because it means they can “self”: a single snail can act as both mom and dad for the same babies. For the strawberries, we are actively investigating the genetics of their sexual system. Hermaphrodites are pretty common in the natural world, but they are off most people’s radar. Frequently encountered animals like mammals, birds, and insects are all either boys or girls. And most people ignore plant sex. I’ve read a few obscure science fiction stories featuring hermaphrodites, but I can’t think of any hermaphrodite character that most people would recognize by name. Where are all the hermaphrodites?

I don’t just mean in literature. A hermaphrodite is your standard-issue, fully functional organism. The default. A device that both transmits and receives information. Why aren’t we all hermaphrodites? It’s even the ancestral condition. The earliest sexual organisms took it literally that the love you take is equal to the love you make. So why would different sexes ever evolve? Why would a perfectly good hermaphrodite give up half of its reproductive capability, potentially reducing its fitness by 50%? This is not the same as asking why sex exists. The reason why many species reproduce by exchanging DNA, rather than by cloning, is explained by hypotheses such as the Red Queen. I’m asking a different question: why can’t each of us exchange genes with any other member our species? Wouldn’t we all have higher fitness if we could do that?

There are at least two reasons why hermaphroditism won’t evolve in a species like humans. For one thing, there are constraints on the way we develop. Male sexual organs are simply an altered counterpart to female sexual organs. The same embryonic structures that become the ovaries, labia, and clitoris in women become the testes, scrotum, and penis, respectively, in men. So there’s no way to build a human with a working set of both the lady bits and the gentlemanly junk, without substantially reprogramming the way we grow. There are of course people whose genitals are intermediate between male and female forms. Such people are known as intersex, not hermaphrodites. Having a single organ that is either a small penis or a large clitoris is very different from having both a penis and a clitoris. And of course there are bisexual and genderqueer people who don’t fit a binary framework psychologically, but they aren’t hermaphrodites either.

Secondly, if a true hermaphroditic person appeared, they would face the same kind of bigotry that intersex people now face. In evolutionary terms, it would be challenging for them to compete for access to either male or female mates. Hermaphrodite mutations would be weeded out by natural selection.

Neither of those reasons imply that a human-like hermaphroditic species would be unsuccessful. They just explain why you can’t get there from here. So why did we separate the teat from the shaft in the first place? Nearly all vertebrates go two-by-two, and those which don’t have reverted secondarily. You need to go back to our pre-vertebrate ancestors, little swimming lancelet-like creatures such as Pikaia, more than half a billion years ago. Somehow, in a population of hermaphroditic critters back then, males and females appeared. Maybe males evolved as moochers on the system, spreading sperm without the cost of producing eggs. Maybe mitochondrial DNA, a special subset of the genome inherited only through the eggs, evolved to suppress the production of sperm, which from a selfish mitochondrial gene’s point of view is just a waste. In any case, it was a trick, a bug in the code that could exploit things for its own temporary benefit. There may have been no direct advantage to the species overall. But perhaps having distinct sexes with particular mate preferences increases the chance that species split and diversify.

So that was that. The ultimate balanced polymorphism got locked in. Testes and ovaries were segregated. The genes involved periodically changed, sex chromosomes arose and were replaced, dudes and dudettes chased each other through ponds, fronds, and proms. But over hundreds of millions of years, we have been stuck with it. The evolutionary equivalent of fifteen minutes of pleasure, nine months and eighteen years of consequence. There’s no going back.

Or is there? I don’t think we’ll sprout two-in-one penis/vulva combos any time soon. But it’s liberating to think that a sexual dichotomy is not some essential arrangement required for life. It’s more like an inconvenient relic. Just like we have blind spots due to a nerve blocking the back of our eye. Or bad backs as a result of standing upright with a skeleton that originally evolved for four legs. Really there are three lessons here for the meaning of human sexuality. One is that the diversity of human sexual behavior is nothing compared to the diversity found across the tree of life. No human can “self”, and no couple enjoys mutually fertilizing each others’ eggs, at least not with their own sperm. So whatever your neighbors are up to, you have no right to call it deviant. Second, the sexual binary is nothing sacred. We can choose what, if anything, we find useful about the concepts of masculinity and femininity. I can be a nurturing father without fearing for my manliness, which has no core essence beyond what I make of it. We are not meddling with the natural order. And third, show a little humility. I don’t care how big your package is, it’s still only half of what a strawberry has.