It's nice the person realized their story was full of holes and then started poking at them. I'm not sure I would have the confidence to tell bad fiction and then rip it apart with equally flawed logic. So brave.
Your comment spawned a delightful train of thought where I imagined all the other fish trying, and failing, to successfully fuck before these two fish stumbled on how to do it.
It’s the most recent common female ancestor of all humanity, as determined by mitochondrial DNA.
Or put another way. If everyone traced their family lines back through their mothers, mitochondrial eve would be the first point where all those lines converged on a single woman
I had a coworker for a while who was maddeningly religious. The type who goes around making it his life's mission trying to convert people -- that is to say, constantly arguing with them, and pestering people about religion incessantly even in situations where it was totally inappropriate. Except he hung around in his own Evangelical echo chambers too much and apparently did not quite spend enough time sharpening his skills arguing with atheists on the internet. Towards the end, before he was fired, I was the only person he'd engage with anymore because I would at least argue with him rather than everyone else who would just tell him to fuck off.
Anyway, one day he came at me with the old, "If we came from monkeys, why are there still monkeys?" I let the monkeys/apes part go. Instead asked him if dogs came from wolves, how come there are still wolves?
He stood there with this vacant look on his face for like five full seconds. I know this sounds like one of those "and then everyone clapped" stories, but he really did. He should have had a little spinning loading icon hovering over his head. He didn't have a snappy pre-memorized comeback for that one. (I believe the accepted creationist answer would be to mumble something about "microevolution vs. macroevolution," or "humans breeding animals is different and doesn't count," or similar.)
Unfortunately, this experience did not seem contribute to actually wising him up in any way.