My reasoning...
Mew: Has DNA of all pokemon, snice it is the ancestor of all pokemon (as King Arceus had said), therefore it can be all pokemon.
Also, what if every pokemon is a mew, just transforming itself into something else? This could be why every pokemon can be tranced back to mew. And since it's been so many years that they've been in a different state (transformed), they've forgotten how to do some of their other moves forever. Thus giving us the diversity that exists in the Pokemon world.
Smeargle: Agrees with DNA. Smeargle's paintbrush tail is a magical thing.
But you can always go back to the reasoning that Ditto is a failed Mew, because it knows Transform and is pink. This couldn't work backwards because Ditto only knows Transform and can't learn any other move, unlike Mew.