every couple billion years the earth passes through some radiation belt that causes people to instantly mutate. Sometimes it works like Magneto's device in X-Men and ends up killing the people (or whatever is living at that time) ala extinctions. and other times it causes crazy monkey things to turn into humans :-p

just a theory...that i made up...as i wrote it...and not a bad one if i do say so myself....maybe ill publish a paper on it...or at least use it as my current belief system

-Darth Simon