While they did eat the whole carcus of animal, it is not primarily corn or grain and not not primarily feet and intestines and the like. In the wild an animal will eat anything to survive. The animal by products you find in pet foods are not of a good quality if you can call those parts quality at all. They are often from animals with diseases and are not fresh. While feeding fresh animal by products would be better then the type that are found in pet foods, even those are of poor nutritional quality. Sure you could feed those things to a dog and the dog migh do alright. But feed better quality food and the dog will thrive and live longer and avoid many health issues. Put it this way. You can eat an average diet and live quite long. But if you eat healthy and avoid things that overall are ok for you, you will live even longer (in general). Since the quality of food has gone up over the past 100 years, we live longer. Obviously medicine also contributes to this but the availability of quality food is much higher then it used to be. Counteracting this though is clearly all the fast food we see today, particularly in north america.