In an early comic [lost on floods, i.d. help] they explained the overlapping chemistry and breathing needs this way. The Preds were descended from Terran lifeforms rescued by a 3rd party extraterrestiral race that they then supplanted, this was long enough ago that atmosphereic comp was diffrent from modern times. Always liked that version b/c the being able to eat etc makes so much more sense. They also implied that they favored Terra as special hunting ground b/c was a sacred site for those reasons. Can anyone remmber that graphic novel?