The very existence of Taken Primevals supports this theory, IMHO. As far as I know, they were "born" that way and not due to being "taken", and are apparently much older than other Taken. I'm a little sketchy on this portion of the lore, so someone else will need to confirm any of this... but I smell a story yet to be told here. Of course, this would then have some implications regarding Oryx's ability to Take, after he "communed with the Deep". When someone or something is Taken, where do they go? And do Taken Primevals originate from there? I think you're on to something with this "fifth race" theory.