We don't know exactly where they came from but the Cabal are definitely from another system, as for the others that is completely unknown. We don't know specifics just that the Collapse and the Dark Age was centuries ago, for the Golden Age we don't the exact amount of time for that either. From the way it sounds some of humanity was actually spared and didn't require Ghosts to save them. We were reborn because our Ghosts deemed us warriors so they revived/reactivated us. Also note that we, technically, are using the Exo introduction (the different races were meant to have different intros) so we could have been an ancient corpse or we could have been recently dead.