Time isn’t a strict progression of cause and effect; from a non linear, non subjective viewpoint, it’s more like a big ball of wibbly wobbly, timey-wimey… stuff. It twists and it turns, and it could have taken a lot of different paths at different points.
Like, for example, with the part of the Doctor. As with any piece of casting, there was an audition process, and a shortlist, and finally an actor was eventually cast – but what if things had gone differently? What if the final choice for the Time Lord had gone to one of the other finalists?
An article on Doctor Who. Nice little listicle. Always fun.