Grains were introduced during the industrial revolution. All primitive diets do not have gluten in the mix. Your gut is a byproduct of the history of your ancestors. This is the reason for the why people have a hard time processing grains in general. As far as a grainless diet. Some patients can tolerate some gluten some can not; every gut is different. I do know clinically those patients who absolutely can not tolerate grains and eat grains have sluggish guts and oftentimes sluggish brains. Excellent book to read is Nutrition and Physical Degeneration by Weston A. Price. It sheds some light on the how did we get here.