On the character select screen it shows a "file photo taken before mutation." VERY brief explanation of what happened. George (ape) was transformed by a science experiment, Lizzy (Lizard) was transformed by a radioactive lake and Ralph (Wolf) was transformed by food.
So they WERE people, and when you "died" you transformed back into a naked person and slowly shuffle off screen. This movie seems to have done away with that premise. Which, honestly, I don't care about.