If we define humans as having a sufficiently sophisticated language, and using a term to reference one's entire species is characteristic of that, then at a stretch we could define the first human being as the child of proto-humans who first invented this linguistic concept.
That would be hard to object to.
> and using a term to reference one's entire species is characteristic of that
But this is easy to object to. People can easily do without a term for the species; it's more common to use tribal names, so that a contrast is drawn between different groups of humans (as opposed to between humans and animals, or humans and gods).
Terms for humans as a species come along pretty quickly, but they're not universal.
What if "human" would be replaced by "ape"?
The logic of these arguments is fine. I just noticed a way to nit-pick your implementation of it.