Generally grains were processed in better ways in times past. An example would be longer fermentation of grains because they didn't have quick-rise bread. So it is important to keep in mind that the average grain you come across today is worse that what people ate in times past. In Nutrition and Physical Degeneration, Weston Price studies two groups of healthy (perfect teeth) remote Europeans that eat grains as a staple of their diet. So grains are not necessarily as bad as you would believe from hanging out on forums like these. It as a combination of grains being poorly prepared, eating 100 pounds of sugar a year, large amounts of vegetable oil, and other processed junk that are truly devastating to our health today.

It looks like there have been some great answers. I thought I'd chime in with some pertinent books if anyone wanted to do some reading on the subject.

Against the Grain: How Agriculture has hijacked Civilization by Richard Manning

1491: New Revelations of the Americas before Columbus by Charles C. Mann

A Green History of the World by Clive Ponting

Anyone have any other suggestions? I'm always looking for new books.

I think DeVany addresses this question in his lecture series. Briefly, it has to do with the quality of your life. I think he calls it physiological overhead. Using wild animals as an example. DeVany demonstrated how wild animals do NOT see the decline in physical capability that you see in domestic animals. They tend to live at near peak performance and then enter a short window of fatigue and diminished capacity and then they die. Contrast that to domestic animals that exhibit a bell curve model of performance and longevity. They reach a peak and then start a slow decent into a further and further decrease in performance. His argument is that you can see the same traits in people.

I had a similar thought when first exposed to Paleo nutrition. Then I remembered, Natural Selection is ONLY concerned with the species. That which is good for the species does NOT have to be good for the individual.

So we may have learned techniques to process grain, and adapted some genes, perhaps ever so slightly, to digest grain. But that doesn't mean any of it has been good for us as individuals.

An interesting (to me ;-) example of this in the natural world is with ants. Once the queen in a colony dies the entire colony is ultimately doomed. And yet the colony continues to function, complete with a battle to the death among the surviving soldier-queens to be the new queen. But a soldier-queen can never be a real queen. A soldier-queen can only reproduce drones. So, Natural Selection has rewarded the colonies that continue to function even just a little bit longer making drones in spite of the fact the colony itself has absolutely no future. A great anthropomorphic story on this is at: http://www.newyorker.com/fiction/features/2010/01/25/100125fi_fiction_wilson?printable=true

Short answer:

Grain is storable. A society can horde it and feed laborers/slaves/warriors. It can also store it for times of less productivity.

Think of the Garden of Eden as an allegory and grain as the forbidden fruit. As grain production flourished, so the concept of surplus wealth thrived. Grain allowed us to exponentially populate the earth in bondage to the soil and its landowners.
It happened around 10K years ago, and as the Bible says, we can never go back to the garden. As grain production occurred so did the afflictions of man, cancer, diabetes , arthritis and other maladies, only problem is early humans died at such an early age, grain consumption was never attributed to disease.

These great civilizations collapsed, most due to the over-extension of their grain supply. Massive deserts followed these civilizations, and environmental destruction was a way of life.

So, in fact, they didn't do that well, they perished because their practices were not sustainable. That should be a BIG hint to us modern humans who rely so much on limited resources and grains.

I think there were a number of things happening. First, the grain we have now, like many other post agricultural products, has been altered. Second, grains can be dried and stockpiled, as can legumes. Third, the stockpiled grains were fed to the lower class in differing forms.

Generic, all-purpose flour is bleached and treated so that it loses it's nutrients an has to be "enriched". Other flours used for bread are grown for their gluten content. Gluten is after all what gives bread it's texture. You can even buy special flour made for bread machines that is higher in gluten than other flours. Today when we think of grain, the first ones that come to mind are wheat, oats and corn, wheat was only a portion of the grain consumed. Other grains, like millet and sorghum, are easier to digest.

Others have stated that grains can be stockpiled. Legumes were also stockpiled. In this way, when times are thin for agricultural societies, there is still sustenance. Legums provided protien while the grains filled the tummy. Bread was also a portable food. Grains, bread and beans could also be carried into battle and on ships for long voyages.

Most of the grain was consumed by slaves and the lower classes. Although bread and some grain was consumed, the upper classes had greater access to high quality meat, especially in the middle ages. The lower class was sustained on gruel, vegetables and the tougher cuts that were thrown away by the upper class. Poor storage methods led to fermentation and the long, slow cooking further diminished the toxins.

I've got a crazy theory that autism is a disease of civilization, but uniquely among the DoCs it encourages technological progress and yet better agriculture. Who do you think invented irrigation? Some bread-eating nerd with allergies, bad teeth, and a weak jawline, that's who.

East Asian cultures are more adapted to wheat with a lower prevalence of celiac, etc; and East Asian cultures are more aspie than Western.

Jared Diamond has some interesting insight to this question. I think the answer is because with grains you can have a higher population density and that allows people to specialize.

His article The Worst Mistake in the History of the Human Race gives some interesting insight. Also his newish book, "Collapse" sort of goes into the fragility of agricultural societies (versus hunter gatherer).

They drank all their grains through beer, they didn't eat it.

http://www.livescience.com/culture/beer-helped-rise-of-civilization-101104.html

With the dawn of agriculture we lost the need to follow our food. We built settlements and domesticated animals. This seems to be progress in most peoples eyes. I mean, if you think of a tribe running around is the wild vs living in a city...well, I can see that. However if you look closer you will see that disease and cavities rose dramatically. Bone density decreased and the average height became shorter. Sound like flourishing? Perhaps the settlements flourished but the people did not.