A human can become one by being infected with a mutagenic virus, one of the main ones being the Will of the Earth. Only Lesser, Devoured and Greater are humans mutated though (Unless you also count Terra and Hamidon), the Razorvine and the like are created by Hamidon.
So they're scientifically based, rather than mystical?
Yup. Sciency-mutanty. Using the old Marvel classification, they'd be Mutates (Scientifically created mutants, as opposed to naturally.)
Ahhh ok....bugger lol.
*goes back to the drawing board* Cheers m'dear 
Does anyone know much about the Devouring Earth? ie how do they spread/can a human become one etc?