I think that they would all steam from the same beginning but each one is a single variation which changes it from then on...like the difference between this universe and the next universe might be me hitting backspace. And then if that were true then maybe if you went back in time you would actually be shifting universes if you could change something and went forwards in time.
I may be wrong, but I think there was actually a startrek(next generation) episode based on a theory like that. I know it is a TV show, but the theory it presented was pretty interesting. For every decision you make in your life, 100s of new paralle universes developed, each one based on how the world would be if you made a different decision.