After an intense millenia of retribution and evolution while waiting for the earth's atmosphere to reset, humans found that the radiation and terra forming that the moon crashing into the earth had done had ruined many lands and now it was crawling with horrible creatures named Immortals. So naturally, humans used their smarts to reestablish the world. Starting small but eventually being able to drive out the majority of the Immortals they could not have a healthy, symbiotic relationship with.