I think that the ancestors of dogs were dogs....some kind of small, dog-like mammal. There's a lot of evidence that dogs' ancestors come from a very loose, semi solitary, but social animal. And maybe their ancestors' ancestors were wolves. There's a lot of controversy on that subject. At any rate, dogs have been dogs for a long time and what is natural to them IMO is what humans have been feeding them (since they've lived with or around humans for a minimum of 15,000 years) and what they could scavange by way of their own talent for scavanging and from moving around from place to place, random animals joining a small group and then leaving frequently.
They and other wild dogs have been more a scavanging animal than a hunter for a long time. Therefore, I believe that although their physical and morphilogical make-up (jaws, teeth, digestion etc) may be more like that of a carnivore, behaviorally, they are an omnivore and their systems are suited for other things besides meat....or just meat. I think they're somewhat flexible in other words. Furthermore, what materials can not be "broken down," is not the entire importance. There is some benefit IMO to some degree of non-digestable material.
Many kinds of wild dogs are seen eating other things besides other animals....fruits, grains etc. My own dogs and I'll bet yours too, will graze on a little green grass. They help themselves to my raspberries and a few other things they find on my property. If given a chance, they'll gobble up a piece of bread or pasta that drops on the floor. They are never the worse for wear. They'll eat dairy things, eggs, you name it. I totally agree that dogs need meat and lots of it. But I also believe that the fact that they eat all kinds of things IS what is natural and has been for a long, long time.
Looking at my own dogs condition and behavior, this makes a lot of sense.
For as long as humans have been eating grain, dogs have been eating it. Now, I don't know for sure the exact temps, but I have been told that the temperature kibble is cooked is lower than what we would cook human grain based foods (pastas, breads, etc.) at to make it digestible, which is where a lot of dogs have problems getting all the nutrition possible from a grain based kibble. I am allergic to wheat myself, but Robert isn't. We added an extra peanut butter sandwich (with sugar/salt free non hydrogenated PB) to Strider's daily diet because it is economical and he really could use the extra weight. A lot of borzoi breeders have been telling us that in order to keep on a little extra weight, especially if you have a very active animal, you need carbs. And that pasta/bread for people was a good source of
quality carbohydrates. It's actually been working really well for him.
Of course, every dog is different, and is going to thrive with different things depending on their own background and what they are adapted to eating. My brother feeds an almost totally vegetarian diet to his akita, which works great for that individual dog. Not so great for every other dog out there. His is based around Avoderm's vegetarian kibble, home cooked, and he rotates one of the diamond natural kibbles into the diet every other bag for variety. Ranger always looks/smells great and acts healthy, he has no health issues at all. Not every dog would thrive on that diet however.
It's interesting to see Striders breeder's dogs and how they thrive on the "Dairy Farmer Diet".
Their family has been working hard at becoming self sustaining. They do it all except grow their own grain because Arizona is not the ideal climate for that sort of thing. Anyway, the dogs are fed raw chicken, raw goat's milk, and goat meat plus boiled rice and scraps. I think the raw goat milk is why Strider's guts are so bomb proof, getting started in life on that stuff must have inoculated them with probiotics big time.
I think a lot of dogs throughout history probably ate milk and eggs as part of their daily diets. Those are two things that would have been available every single day on farms, and are not easily preserved for future use. I know it was always two protein sources we always had extra for the animals when I was growing up. We always had a bucket of milk for the pigs, and a bucket for the dogs after the calves had drank their bottles, and there was always plenty of eggs. Each dog got at least an egg a day, if not more. Then you have farm vermin, like rats, rabbits, foxes, etc. if they could catch were probably eaten too.