I have some ideas, maybe wrong who knows, about how humanity was years ago. Lets say that the first dogs/wolves approached humans back then. They could be nomads, so hunters so obviously they would feed meat etc or early days farmers, but then what use could they have for dogs as farmers? guarding duties maybe? so I have to accept that from these farmers' dogs the dogs evolved into better digesting grains?
Also, later on, they could be owned by upper class, so meat they would eat or lower class, income etc, so could they really keep dogs and feed them? In full honesty I can never imagine a period in humanity's existance that there was a surplus of grains etc to feed dogs. Humanity was poor except from royalty plain and simple, they could throw a bone maby but, lets say, bread? I do not think so.