There are many theories that point to the fact that most people weren't designed for digesting grains. You may find this website interesting:
http://www.proteinpower.com/drmike/uncategorized/obesity-in-ancient-egypt/Excerpt:
Ten or twelve years ago we wrote in Protein Power about the data contained in the vast amount of ancient Egyptian mummies. We pointed out that several thousand years ago when the future mummies roamed the earth their diet was a nutritionist’s nirvana. At least a nirvana for all the so-called nutritional experts of today who are recommending a diet filled with whole grains, fresh fruits and vegetables, and little meat, especially red meat. Follow such a diet, we’re told, and we will enjoy abundant health.
Unfortunately, it didn’t work that way for the Egyptians. They followed such a diet simply because that’s all there was. There was no sugar - it wouldn’t be produced for another thousand or more years. The only sweet was honey, which was consumed in limited amounts. The primary staple was a coarse bread made of stone-ground, whole wheat. Animals were used as beasts of burden and were valued much more for the work they could do than for the meat they could provide. The banks of the Nile provided fertile soil for growing all kinds of fruits and vegetables, all of which were a part the low-fat, high-carbohydrate Egyptian diet.Another snippet:
Were the nutritionists of today right about their ideas of the ideal diet, the ancient Egyptians should have had abundant health. But they didn’t. In fact, they suffered pretty miserable health. Many had heart disease, high blood pressure, diabetes and obesity - all the same disorders that we experience today in the ‘civilized’ Western world.Check out Dr. Eades' Book "The Protein Power Life Plan". It's a really interesting take on how we evolved to eat.