Oooh, Yamamoto has been Captain-Commander for 1000 years, and he's the 30th. From that it can probably be extrapolated that Soul Society existed before human civilisation. :3 :3 (Unless all or some of the first 29 commanders went in quick succession).
I really wonder what the proportion is of Shinigami who were born as spirits in Soul Society to those who became Shinigami after having died as humans. I mean, what with the whole balance between worlds bit, Seireitei dwellers can't exactly multiply fruit-fly fashion. Theoretically, the population of Soul Society (both Seireitei and Rukongai) is comprised of everyone who has ever died (but has not yet been reborn as per Bleach canon), and then there are the 6+ billion (and rising) people on the planet. But presumably, for every human born, a soul from Soul Society reincarnates, so does that mean that a new Shinigami must be born right then? I think I need to make that flowchart after all. Illustrated with Rukia's bunnies.
Is it just me or does Aizen look surprised at the revelation that Yamamoto is no spring chicken? o.O Really, dude, come on.
Also, Yamamoto says that a Shinigami stronger than him has not been born in those 1000 years -- I guess Ichigo doesn't count, since Ichigo was not born a Shinigami; he became one in Urahara's Pit of Hell. xD
In other news, Wonderweiss continues to be creepy and wrong. Someone please kill it now.