The Muggleborns come from the natural evolution of humans--like Jack, they're further along the evolutionary path than Muggles.
I want to write a fic about this, but I think it'll have to wait until the theory has straightened itself out in my head. I also need to re-watch "Avalon" (both parts) first.
Anyone have any thoughts on this? (Surely I'm not the only one who's had this idea.)