Every single race from every single planet has some kind of super power, except for humans from Earth.

This is obviously because this is a prison planet with something, probably a big black monolith floating in orbit, that prevents humans from getting in touch with their super-powered side.

However, after millennia, this thing has gotten weaker, so like, any old excuse will give someone super powers. (let them get in touch with their super-powered side) I mean, like a really dense meteor (that didn't completely damage the earth) can be picked up by a teacher and give him shrinking powers.