Wiithn the Pokemon canon, there was myths in Sinnoh that mentioned that Pokemon and Humans used to be one and the same, or atleast intermarried (US releases just removed the reference and made it more like "ate at the same table" instead). i just assume that Humans basically diverged from Pokemon and intermarried amongst their own kind, eventually being a distant cousin.
Fanonwise, i think this shows itself with Aura and Psychic power-users, because really, they seem like odd skills for Humans to have in a world where there's super powered animals, but being genetic throwbacks to pokemon would make sense.
From a self-taught basis, think Humans learned how to make cement from Conkeldurr...and to Pokemon they learned how to do Toxic from human ninja techniques...which is strange for Pokemon like Wigglytuff, which might imply that Humans could actually create poison themselves (assuming Jutsu) or if it's a species-wide ability that only became teachable once Humans "reverse engineered" it.
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