i think that came along a bit later as it became apparent they could be used that way.  but getting food ,which the dogs did play a large part in, was always a huge thing for early man. the dogs and  people together hunted but also dogs jaws could break bones open with the rich marrow inside way more efficiently than man could smashing them apart open. i would not wanted to been one of the first people to train the dog to let the bone go to a person.  i imagine pups  just learned and adult strays got man handled. dogs are naturally protective of their pack and turf so them wanting to  fight was a pretty natural thing .  i love this subject... thought is that women first domesticated dogs by actually nursing found pups and a natural bond that happened brought dogs right into the tribe/pack...