You do realize that man lived and survived centuries before any grain was ever raised,right? We were not made to survive on carbs and our metabolisms were not set up to do that.
Humans without agriculture - outside the arctic circle - tend to derive the majority of their calories from non animal sources - those sources tend to be high in carbohydrates, nuts, fruits, tubers, etc.
You do realize that for centuries before man figured out how to harvest grain, he still knew how to dig up a root or tuber and eat them, right?