You're right. It is pretty interesting to say that Mew is the ancestor of all Pokémon. As I've heard when I was a child, Mew is the ancestor of all Pokémon because he knows how to learn all kind of attacks.
I'm wondering why Smeargle can learn all kind of attacks too. It makes me confusing. Maybe Smeargle is one of the oldest Pokémon.
A part from that, if we don't conciderate Arceus and Mew, which one would be the oldest one? I think this question could be tough to answer !