Wheat, rice, millet, corn/maize, sorghum, barley, oats, spelt, rye 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 in the Palaeolithic. 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 toxins - particularly leptins - toxins that evolved in the seed heads of grasses which enabled their survival in the face of predators that sought their concentrated energy. This means that any dietary experiments to treat a grain-related intolerance or allergy should exclude (a) all grains, not just those grains containing gluten and (b) all meat, eggs, poultry, farmed fish etc. that have been fed on grains.
Newcomers to the Palaeolithic 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 Palaeolithic diet. How do we know this? 
In the Neolithic, grains began to be farmed: sown, cultivated, harvested, transported, hulled, winnowed, milled and baked. The seed was selected and sown. In the Palaeolithic, in contrast, seed was not sown, cultivation would have been rare and cursory. Harvesting would have been confined to about two months in late summer and would have been done by hand - 'gathered' by hunter-gatherers. The seeds then were smaller than grain seeds used today and the heads had far fewer seeds, the plants would have been mixed in with other grasses and would have been further thinned by humans' competitors: seed-eating birds, grazing animals, locusts and weevils. For humans to use any food as a staple, it has to provide more calories than it requires for its production. Palaeo-anthropologists refer to this as 'optimal foraging'. Foods can, however, also be used as flavouring, as components of rituals or as seasonal indulgences without regard to cost/benefit ratio and so pestles and mortars can be found in Palaeolithic societies, millennia before the beginning of the Neolithic age. The grains that survived longest were those containing toxins which deterred predators. The predators themselves evolved with ruminants emerging among them; they used gut bacteria at the beginning of their digestive process which render toxins harmless and enable the ruminants to thrive as grazers. Humans had no way of neutralizing the protective toxins until they developed cooking. Humans did not always have access to fire; a few cultures learned to make fire, but others used natural fires as a source of combustion and nurtured this source over many months, taking it with them on firesticks and they moved about nomadically. Their cooking method was as a 'damper' tossed into coals or spread out on hot rocks beside a fire. Having considered the time required to produce a meal of grain, you can see that hunter-gatherers would generally prefer to eat meat from larger mammals than gather and prepare grains.
1. Todd Moody points out here that we don't - and can't - really know: "The problem is, our theories about actual paleo diets are grossly underdetermined by the evidence. That's why we must so often resort to inferences like, "Surely paleo people didn't walk by apple trees without taking some" and so on. Or we use our imagination to reconstruct how they used fire, or why they didn't. And of course, it's not just we who do these things. The scientists do it too. The greater the degree of underdetermination of theory by evidence, the more contentious the science, and paleoanthropology is a very contentious science, with many bitterly disputed claims. They are bitterly disputed because they're so hard to settle." We would add to Todd's observation by saying that they are contentious because they have real implications for our view of ourselves: Can I be blamed for my illness? How can I justify my vegetarianism? Should I prefer a natural remedy over a modern allopathic treatment? Back to text
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