Complex sentence: At the time the original series began, there had been no contact between the thirteenth tribe and the Twelve Colonies of Man, and both the existence of Earth and the thirteenth tribe were largely considered a myth.
Split into simpler sentences: At the time the original series began, there had been no contact between the thirteenth tribe and the Twelve Colonies of Man. People of the Twelve Colonies of Man generally considered the existence of Earth and the thirteenth tribe a myth.