I've been toying with the idea that the Traveler put the consciousness of an ancient warrior long since gone into the body of an EXO, and that the other races were warriors more recently deceased.

[quote]Only thing wrong is that then how were they dead in beginning of game if they r immortal?[/quote] Doesn't mean they weren't damaged. If they couldn't repair they just be an inert Exo until the Ghost resurrected them.