Some coincidental thoughts from all the ideas turning up: I dug around to find the original Zoroastrian creation myth (here it is, in more detail than some of you may care for: http://www.avesta.org/pahlavi/bundahis.html ). And I was noodling around with wondering why Wolfe, a professed Catholic, if perhaps not conventional, chose to use this model, which ignores Original Sin in favor of a thoroughly dualist universe. And I though, well, Urth is dualistic, but Briah is not. And then I thought---aha, the series metaphor for Original Sin is the black hole at the heart of the Old Sun! Which Severian, the Savior, obliterates.
I must say, this black hole had escaped me completely, though mantis thinks it is so obvious as to be beyond comment. But, though I still don't know how it got there, it seems a very powerful and important image to me. And I add to it Blake's equally powerful "The Sick Rose." Blake was, of course, a religious maverick bouncing off Protestantism, and may not have been thinking of Original Sin at all, but the poem is so apt as to be breathtaking, and we know that Wolfe was quite consciously using rose imagery throughout. Here it is: http://www.mindspring.com/~cfeldman/sickrose.html
-alga-