There is no doubt whatsoever that we cannot be a vegetarian species. From at least the time that Homo erectus appeared in the cold Eurasian continent some 500,000 years ago, we must have lived on and adapted to a diet almost exclusively of meat.
All this evidence points to our being pure carnivores, as are the big cats. However, we are a remarkably successful species. It is unlikely that we would have been quite so successful if we had been forced to rely on only one source of food.
It is obvious from archaeological remains that we tended to be more opportunist eaters. We hunted and ate meat primarily but, if meat was in short supply, we would eat almost anything - so long as it did not require cooking.
This still precluded some of the roots and most of the legumes and cereals that we eat today. When meat was in short supply, we got our protein from nuts and ate fruits and berries. During our evolution, therefore, when we lived well, our diet was high in protein and fat: during lean times it was richer in carbohydrates.
So, our ideal diet, the one we evolved and adapted to, must also be one which is high in proteins and fats, and low in carbohydrates.