It seems that whenever you get a group of dog breeders, dog lovers or dog owners together and start discussing dog diets there is a natural split in the group. There are those that believe that dogs have to be fed as their ancestors were 30,000 years ago or more, and those that believe that science and research has developed more nutritionally comprehensive diets that are better suited for health. To top off that disagreement there is also the whole carnivore or omnivore debate which is just as sensitive for many in the dog world.
Historically dogs all developed from a wolf like ancestor. At this time they did only eat meat and a small amount of grain, grasses or fruits and vegetables that may have been found in intestines of their prey. This was a minimal amount to say the least. However, there is no indication in the fossil record if these same animals ever elected to eat grasses, grains or other forms of foods. There is no doubt that the teeth are those of a carnivore, not a herbivore, but they also are different than the teeth found in modern dogs.
As dogs began to hang around human encampments and tribes it is likely that they also consumed some of the waste food material thrown away. This may have included cooked grains, vegetables, fruits and of course different types of meats including fish that may have been very rarely consumed in a wild diet. In addition cooked foods also became much more palatable over time to these camp following dogs.
From this point the change in the domestic dog's diet really didn't change much although they were more closely aligned with human food consumption. In the early settlements the dogs ate all types of table scraps and managed to survive long enough to reproduce and continue the lines. However, these same dogs also would have had access to raw meat through hunting, at least for several thousands of years.
The first commercially produced dog food wasn't available until the mid 1800s when an American, visiting London, saw how the stray dogs at the shipping yards clamoring for scraps and hard bread thrown overboard. He came up with the idea of bone shaped hard biscuit that was all vegetable matter with beef blood for flavoring. In 1950 his company, which had by then expanded into different types of feed, was purchased by General Mills. The major ingredient in all of these early feeds was grain, it was both a bulking agent as well as a way to reduce costs of production.
Over time and multiple generations the dog's digestive system adjusted from living completely off of protein to living off of a combination of protein and carbohydrates. Now, with an increasing emphasis on protein in the diet from meat sources, the digestive system may again be adjusting to what humans decide is best for their pets. The good news is that research shows that both raw feeding, with proper balance and understanding, as well as the high quality commercial dog foods can allow your pet to life a happy, healthy, nutritionally balanced life.