Although I wrote about this topic quite recently (“Why Two Sexes?” Feb. 15, 2024), I’m revisiting it due to increasing interest — and confusion — around gender politics. While one’s gender can be as fluid as our culture is diverse, biological sex isn’t. Eukaryotes like us are born male or female, depending on the size of our sex cells (gametes). There’s no middle ground, but perhaps not in the way you might think. So, to review:
Sometime between the happenchance of the first simple cell on Earth and today, sex happened. It had to wait for the emergence of eukaryotes, offspring of bacteria and archaea. Eukaryotes are formally organisms with discreet nuclei, but functionally they distinguish themselves by their ability to eat other organisms. Before sex, bacteria and archaea reproduced by cloning, so, other than the odd mutation, parent and child were identical. Then, about a billion years ago, eukaryotes changed the rules of the game with sexual reproduction, in which the genes of two organisms combine in a random fashion, so that each offspring is different from both their parents.
From a bacterium’s point of view, this makes no sense whatsoever. Why mess with flighty, unpredictable offspring if the whole point is to preserve your genetic integrity? You’ve got a worthy gene that comes from a long lineage, one that’s clearly successful, having been passed down through a zillion generations; if it ain’t broke, why try to fix it? For faithful reproduction, the asexual route is the way to go.
But if, instead, you’re interested in maximum variety — to play with the endless possibilities offered by random combinations of genes — let’s hear it for sexual reproduction. Many, probably most, offspring resulting from a random mixing of their parents’ genes won’t be any better off than their parents. But chance will result in some being better adapted to their environment, thus able to reproduce more efficiently than their siblings. With nature experimenting with so many possible combinations of genes, you might expect surprising outcomes. And indeed, all the mixing and matching has led to the wild variety of eukaryotes on our planet today, from single-celled picozoans barely a tenth of an inch across to 100-foot blue whales and 390-foot-high redwoods.
Why two distinct sexes? Almost all eukaryotes have male and female forms, one small and mobile, the other large and stationary. It’s this size difference that defines biological sex, not anatomy or XX vs. XY chromosomes*. You can see the advantage in the accompanying sketch. If all sex cells were the same size, as was probably the case early in the evolution of sex, they would have had to multitask, wandering around looking for a partner while schlepping a bunch of nutrients to help their offspring to survive and grow.
In creating the sperm and the egg, nature stumbled on a way to separate these counterproductive assignments. The male of the species — whether a fungus, plant or animal — produces many mobile sperm, tiny packages of DNA with some means of propulsion, while the female focuses on creating big, nutritious eggs, ready to be fertilized. This offers some species — most mammals, for example — a say in their choice of mates. Whether it’s a male thrush’s seductive song or a woman’s prominent breasts in a dating app photo, the idea is to attract and mate with the healthiest partner around.
Note that males have to compete against their own sex because there are way more sperm than eggs: In humans, a woman releases about 500 eggs in the course of her life (from her original store of more than a million), while men release up to a billion sperm in each ejaculation. Perhaps it’s not crazy to think that some behavioral differences between males and females, particularly in our own species, ultimately come down to the difference in size between our sex cells. Happily, a million or so years of socialization have given us the ability to choose empathy and kindness over our instinctive urges.
*In humans, rare chromosomal variations such as 47:XXY, i.e. an extra X chromosome for 47 chromosomes in each cell, usually cause infertility. Otherwise, individuals with variations from typical XX or XY, including intersex people, still produce either sperm or eggs, not a third type of gamete. In terms of gametes, there’s no “third sex.”
Barry Evans (he/him, barryevans9@yahoo.com) blames all his crass male behavior on his genes.
This article appears in Cowbells and Crab Hands.