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
Imagine being the last (female, strictly matrilineal) descendant of one of her daughters: "If I die without a daughter, the line of Alice will end and that bitch Caitlin will become the new Eve!"
Hell, imagine fighting a secret war to extinguish the other line - now there's a "secret society conspiracy" plot if I've ever seen one.
There's also a Y-chromosomal Adam, but they lived many thousands of years apart. That seems very counterintuitive, but when it is explained to you by someone who knows what they're talking about (i.e. not me), it does make sense.