Grains
Wheat, rice, millet etc. Although grains were undoubtedly nibbled when Australopithecines, Paranthropines and the Homo species came across them, these grains would have been smaller than the highly-selected grains that have become our staples after 10,000 years of agriculture. So, yes, grains would have been eaten but they could not have been a significant part of hominid diets. People today who are gluten-intolerant are manifesting an inherited Pleistocene absence of full adaptation to grains containing gluten. But it is not gluten which leads to the greatest incompatibility between our physiology and grains; gluten is not, after all, present in all grains. The main problem derives from leptins. This means that any dietary experiments to treat a grain-related intolerance or allergy should exclude all grains, not just those grains containing gluten.
Newcomers to the Paleolithic diet, particularly those who have adopted the diet after a record of failure with other diets, often agonize about excluding grains. There appears to be a pervasive pro-grains cultural conditioning which is hard to shake off. This conditioning extends from pre-history and features in ancient texts and the various religious traditions as well as permeating contemporary images of rustic, wholesome, good health. The evidence from human evolution is unambiguous: grains were a negligible part of the Paleolithic diet.