I am not convinced that there wasn't a 13th tribe. Imagine a mass exedous from Earth. Kobol is settled. 13 tribes are formed, probably to aid the governing body. Eventually, the "gods" make life on Kobol undesirable. A second exodus occurs, but there is some disagreeement among the tribe leaders as to where to go. 12 tribes leave to form the colonies, the last one returns home to Earth. Perhaps at the time the second exodus occurs, Earth is considered a myth, as it is today (in the BSG timeline). This might explain why the other 12 didn't want to waste time trying to find it. Interesting theories though. Look forward to reading more as you have them.