Why does the same thing happen to a piece of food after it has been swallowed by humans of different ethnicity? Why is the anatomy and physiology of the gut virtually identical in a Chinese and an African? Why do all human have the same endocrine system and metabolism? You know the answer: because we share an ancestor from way back when. The experts estimate that our latest shared ancestors lived around 200,000 years ago in Africa.
Before that, during millions of years of evolution, the digestion and metabolism of our primate ancestors had been fine-tuning how it uses the available food substances in the most beneficial manner possible. Nobody would doubt that the best food for the human species would be the kind of food that was available in those days, rather than those that were introduced long after the construction of our physiology. Funny that nutrition authorities never say it loud.
Our primate ancestors have been consuming fruit, vegetables, nuts and insects for 50 million years or more. Meat was successively added, with a probable increase around 2 million years ago. Underground storage organs (roots, tubers, bulbs, corms) possibly become staple foods 1-2 million years ago. The variability was large: single plant foods were rarely available in excess, which reduced the risk of adverse reactions to bioactive substances in plant foods.