Evolution of the species. On the humorous side: They tried being free weights, but it didn't work out. More on the development and domestication of dogs: Our dogs descended/evolved from the Grey...
Dogs running loose may have been having to fight for their lives, fight over food, or fight over territory. They would fight your dog if they thought it was taking away what it needed like food,...
in general, there could be two realistic answers to this.
The first as a joke ... none, dogs don't have feet they have paws.
The second answer forgiving the feet/paw issue and presuming you meant...