In an on-line book, I read:
The study of comparative anatomy and the different natural diets of animals in the wild indicates strongly that the natural diet of early humans consisted predominantly of sweet fruits, and that even though millions of years have passed, the anatomy and digestive apparatus of humans has not changed and is therefore still best suited to fruit as the most suitable food. That this opinion is not just idle speculation can be quickly proven by any sick person who can break the addiction to our modern taste-stimulating foods and go on a diet of good quality fruit for just a few days. Of course the human digestive system is quite capable of handling foods of animal origin, including animal fats, but in only very limited amounts can it do so without strain, even when the foods are eaten raw as intended by Nature.
Thus it can be surmised that the ideal diet for man is one mainly of sweet fruits supplemented by various berries, green nuts, shoots and occasionally small amounts of foods of animal origin, all eaten raw. This is the sort of food eaten by man's closest relatives in Nature, the orangoutang and chimpanzee, both of which have an anatomy and digestive system almost identical to man's. Neither of these animals in the wild display tooth decay or any of the other diseases common to humans, but soon do so if kept in captivity and fed cooked and processed food.
This makes a lot of sense. I'll expand on this a bit latter.