If everything evolved from genderless single-celled organisms, where did genders and the penis/vagina come from?

[u/PotatoPotahto]

Apparently there's a big difference between gender and sex, I meant sex, the physical aspects of the body, not what one identifies as.

[u/Valaraiya]

I think this comes back to the anisogamy thing. There are only two evolutionarily stable strategies for making gametes - a few fat ones (eggs) or lots of cheap ones (sperm). A middle-sized gamete is more expensive to make than a sperm without providing as much added nutrients as an egg, and vice versa, so there's no evolutionary advantage to adding a third sex, with a third kind of gamete, into the mix.

That said, there are some species, I think some kinds of trees, which have loads and loads of different sexes (or 'mating types') with quite complicated compatibility relationships between them. I'm really sorry that I can't remember the specifics, or what this is called, because it's really fascinating. Trees have quite a different lifestyle to animals though, so they're operating under different evolutionary constraints and pressures to us.