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There's also the issue of Ghost type Pokemon. Now Pokemon itself tends to be a little vague about the issue. But it seems that at least some of the Ghosts are created from humans.
And then there's the case of that human who turned into a Kadabra.
What does this tell us? Well the Kadabra kid was apparently a gifted psychic himself. So the psychic powers altered him to become a different being.
One could surmise that what separates humans from Pokemon is the fact that each rests on a different "frequency" but when that frequency is altered. I.e attaining psychic powers, or dying. You then become a Pokemon.
At some level, perhaps all humans become Pokemon of different types. It makes perfect sense when you consider how intelligent these Pokemon can be. How they can understand us even better than the dogs in our world.