After the Nanite event every living organism was infected with dormant or active nanites. Those organisms with active nanites were transformed and later called Evo's. Most Evo's are agressive and gain overgrown, grotesque apendages. Plant or animal like mutations are most common though mechanical transformations have been seen as with the cases of Rex and Van Kliess.
Evo's posses a variety of powers from brute strength and super speed to teleportation and flight. While most Evo's are stuck in thier mutated state and use their powers due to the nanite tranformation a few Evo's can hide these powers if they so choose. Rex, Circe and one Evo held captive by Breech can stay in a human form and use thier Evo abilities as they choose.