No caveman ever ate grass by choice over meat. Fruits nuts and berries were occasionally eat. Roots were only eaten when the hunt was poor. Agriculture and grains were only introduced in the past 3.5 to 10 thousand years, an insignificant blip on the evolutionary timeline. Carbs and sugars make you hungry and fat. You should eat mostly meat and it should be fatty meat. Once you get on a natural evolutionary diet you will become lean and ripped.