We evolved from a primate very much like Chimpanzee, some 5 million years back. Chimpanzees are omnivores, they eat mostly fruits, but some bugs, ants etc, and once in a while some meat. We evolved with that basic diet. During the next 4 million years we developed tools which allowed us to become hunters and add more meat to our diet.
During the last million years humans developed the technology to control fire. This was a major step, in our evolution. It allowed us to cook the meat, and made it easier to digest it. We could also cook the tubers and other previously indigestible matter.
During this million years our diet became predominately carnivore, as meat became easily digestible. It also increased our survival, by allowing us to eat tubers when meat was not available. The simplified and plentiful diet, allowed us to increase our brain size. It also reduced the need for the large colons of primates, which resulted in a reduced pelvis.
Our diet was predominately carnivore, when meat was available, and starchy tubers when it was not available. Starchy tubers were not completely nutritious but provided us the required energy, in times of need. Our main source of nutrition for all practical purposes was meat.
Consequently, our body prefers a meat and fat diet, but can handle a large amount of carbohydrates. No other animal can handle such differing diet. This also allowed us to handle the grains, when we started to farm.
Grains are not our natural diet. We only began farming some 10,000 years back. Archeological evidence shows that whenever a group of hunter gatherers adopted agriculture, they lost stature, lost a healthy bone structure, and showed dental troubles.
Genetic adaptation requires selection pressure. The grain diet does not do any damage in the early years. It does not prevent a person from having children. This does not give any selection pressure, to adapt for the grain diet. Still there must be inevitably a very small amount of adaptation, but its not enough, to let us handle this diet.
I will be writing more posts about what Evolution and Science have shown about our diet, and our medical establishment has ignored.