In The Hills Have Eyes/The Hills Have Eyes II; How Did The Mutants Become Mutants?

I've seen both movies but it's been a while so I forgot. And I don't even think I understood quite how the mutants got to be mutants when I watched them. So like at the beginning of the first one, it shows like explosions. So is that how it happened? I don't know...I'm just confused. Thanks in advance. (: