Grain is unfit for human consumption. The primary diet of grazing and hoofed animals (and some birds) cannot be digested by omnivores, such as humans. And when it is digested in a refined form, it is not nourishing.
Sometime during the Bronze Age, and likely during one of many famines, early humans figured out that a small amount of edible bran and germ existed within a grain which could be extracted ("unbolted") by grinding the kernel between rocks and blowing off the inedible husks (chaff). The unbolted portion of grain was called "meal" (because it was--in a pinch--edible) and the unbolting process was thus called "milling." This was a monumental discovery, since grains can be stored during famine.
Later on mankind discovered that the meal could be cooked with water and formed into a gruel or porridge. And when ground finely enough into a flour, it could be baked into bread, a portable food. Finally, the Chinese figured out how to make noodles (with rice flour) and the Italians adapted the Chinese technique to make pasta from wheat flour. Civilizations were thus defined as the cultivation of "cereals," the starchy (high-carbohydrate) grains and grasses grown for human consumption-- wheat, rice, rye, oats, maize, buckwheat, millet, quinoa, etc.
This brief history is only mentioned to illustrate that cereal products (meal, gruel, flour, bread, and pasta) are the world's oldest processed foods. And the fermented gruel (beer) is the world's oldest beverage. Keep this in mind the next time a diet "expert" advocates the consumption of whole grains in favor of processed foods.
Another fact to remember is that the consumption of seeds of grass (grain) by humans is very recent in human history. Also, breakfast cereals have only been consumed widely since WWII (when real food was reserved for the soldiers).
The only problem with "hoofed animal food" (cereals) is that they are nearly completely devoid of nutrition compared to "people food" or even "rabbit food." While cereals helped to prevent famine, diets overly reliant upon them resulted in the growth of societies of stunted, malnourished individuals. Cereals also contribute to obesity and diabetes in societies which originally subsisted on native plants and game. They do provide some roughage (to fill the stomach) and lots of carbohydrates (energy for labor), but little else. There is more fiber in an apple or pear than in a loaf of bread or a bowl of breakfast cereal, plus a lot more nutrition with fewer carbohydrates. (Maize, or corn, is the most edible and contains the most protein, fat, carbohydrates, and calories of all the grains.)
Producers are aware of the nutritional shortcomings of grains, and that's why in the U.S., cereal products are sometimes either enriched (with vitamins and minerals) or richened (with fats) or both.
Europeans, rather than enriching their flour, prefer serving fats, proteins, and vegetables with breads and pastas. Italians, for example, always enrich a humble polenta or pasta dish with dairy, protein, vegetable, and olive oil ingredients. The Scots and Irish, lovers of oats (a grain considered animal feed by the English), add scraps of meat to a thick oat polenta and call it Haggis. And German immigrants who farmed corn in Ohio and Pennsylvania (and oats to make beer) added pork scraps to corn polenta to make scrapple, or to oat polenta to make goetta.
What fattens hoofed animals and is avoided by omnivores should therefore not be eaten by humans except to avoid starvation or in small quantities in a normal diet. They should defininitely not be the base of the Food Guide Pyramid, promoted by the U.S. Department of Agriculture to support the production of grains and cereals.