Evolution of dogs
Did all of the different breeds of dogs start with one dog?
Yes. Evolutionarily, all dogs began as wolves. They evolved somewhat on their own into slightly different breeds based on location and variation, but really began to develop into the breeds we know today when they were domesticated by humans and certain traits became desirable to different people. This is also why many purebreds have health problems; they were bred to have features unnatural to them, and now they are suffering for it.