Have you ever looked at the stomach of a gorilla? Their abdomen is almost twice the circumference of their chest. They are definitely pot-bellied. One of the evolutionary differences between humans and other great apes was the reduction of the length of the intestinal tract. This was achieved through bipedalism and tool use. By hunting and fishing, humans could get nutrient-dense meats and fish instead of being limited to fibrous, hard-to-digest vegetables.
The introduction of fire for cooking 300,000 years ago also influenced the reduction of the length of the human digestive tract. Cooked foods are softer and more digestible than raw foods, and cooking improves longevity by killing pathogenic bacteria and parasites that can cause diseases. The Neanderthals that preceded us had a bulky mid-section that was advantageous because it kept them warmer during the cold weather of the ice ages. The use of fire by hominids for heating and for cooking was well established by the time that modern humans appeared 160,000 years ago.
If we were limited to eating raw fruits and green vegetables, we would need to eat a lot. To get about 2000 calories, it would be necessary to eat 3 apples, 2 oranges, one whole head of lettuce, 4 medium carrots, 5 figs, 4 large tomatoes, 4 bananas, and 1 cup of walnuts. The walnuts contain about one third of the total calories, but all this food only has 39 grams of protein which is about two thirds of what an average human needs. Modern vegetarian diets reduce the amount of food needed by using beans and grains that must be cooked, but are more nutrient-dense than fruits and vegetables that have a lot of water. The inclusion of eggs, milk, meats, and fish further reduce the quantity of food needed in the diet. Some vitamins, such as Vitamin B12, are found primarily in meat, eggs and dairy products. The proper balance of animal products, vegetables, and fruits provides the complete nutrition required for modern humans.