There are a number of documented cases, apparently, of tribes being discovered that ate basically meat, and few, if any vegetables (it seems to be a common notion that vegetables are not fit to eat, except during times of famine). We are fortunate to have observed their transition to the standard Western diet and the onset of all the diseases of metabolic disorder.
One comparative study of the Maasai versus the Kikuyu noted the much greater health of the Maasai, on their diet of animal products. The Kikuyu were mostly vegetarian and were shorter in stature, frailer, and subject to a host of chronic diseases. George Mann noted in the 1960’s that the Maasai who remained on the traditional diet retained their health, whereas their relatives who migrated to the cites and adopted the Western diet were just as obese and diabetic as the rest of us.
Dr. Michael Eades has a fascinating lecture on the health of the ancient Egyptians after the discovery of agriculture, based on autopsies of mummies, portraits, and translations of various documents.
This is not to deny the reality of healthy peoples who ate vegetables. But the constant theme running through their stories, as far as I can tell, is a notable lack of sugar and refined grains in their various diets.