That depends on the ability, anonymous. In the case of unusual feats of a physical nature, some of these cases were simply a matter of individual training. For example, trainers who work alongside fighting-types such as machamp or infernape sometimes grow stronger or more agile than humans who don’t, simply because of the nature of their training. On the other hand, those that suddenly develop special abilities—such as, for example, the people who gain mild psychic abilities or heightened intuition from years of training with psychic-types—are less understood or explainable. The going theory is that our bonds with pokémon become so strong that they unlock hidden potential within every human. That is to say, every human may be psychic; it’s just that we can’t access those abilities until we bond with pokémon.
Which leads me to those, like Sabrina and aura users, who were born with that potential already unlocked. Where did that potential come from? Where did we as a species come from, and what are we if we’re members of the animal kingdom but possess such strong links to pokémon? Those are some of the greatest mysteries of our world, the kind that every researcher wishes to answer.
Personally, I believe that the potential we all have is indeed the vestiges of our heritage—though not necessarily that we were ourselves pokémon once. (I’m open to that theory, but we’re too genetically different. That’s why humans belong to an entirely different taxonomic kingdom from pokémon.) What I mean to say is it may be an evolutionary adaptation that’s meant to better equip us for life in cohabitation with pokémon. After all, no animal lives closer to pokémon than the human race, and I mean no arrogance or speciesism by saying that. It’s simple fact: whereas it’s true that all other animals share habitats with pokémon (sometimes even working together to survive), humanity’s relationship with pokémon runs deeper, to the point where we often form lifelong partnerships akin to the bonds human best friends share with each other. (I know that some pet animal owners will take exception to this comment, but I mean to say that tamed pokémon are seen as very nearly equal to their human partners.)
So because of this relationship to pokémon, our species has gradually evolved to keep up with them, so to speak. We’ve evolved to become adaptable and easily changed, with vast reservoirs of potential within every one of us, just waiting to be unlocked to match the partners we choose. That, in my opinion, is part of what makes humans so fascinating.