There are lots of reasons. An important one is this: Grain allows storage of food over months or years and relatively higher concentrations of people under one central ruler or religious ideology. That means that grain-based civilizations can conquer and expand into areas previously inhabited by hunter-gatherers. Eventually, the HGs are pushed out or assimilated.

You can also make alcohol with them.

Grains (particularly gluten containing grains) stimulate the opiate receptors. So we started eating grains because they made us high. And they were so addictive that we started agriculture so that we could always have them.

I watch a lot of bad TV (it's a way for me to relax after thinking hard all day at work - so don't judge me), two shows that I used to watch were A&E's "Intervention", and MTV's (I think) "Dr. Drew's Celebrity Rehab". It's interesting to watch them because they deal with both sides of the drug addiction - trying to convince someone they have a problem and need rehab, and then watching the rehab process. After watching a number of these shows you can easily see a pattern of behavior in addicts. What's really interesting is that is see these same patterns in people when both trying to convince them to give up grains and then what happens to them as they're going through withdrawal. It's eerie how similar it is to drug addiction.

They store well.

I was thinking about this earlier - a lot of the things that store well aren't very good for us. Someone mentioning peanut butter elsewhere started the thought. It seems like most of the things that are best for us are highly perishable! Quite a conundrum.

Well, there's a couple assumptions here that don't necessarily pan out. 1) That everyone feels like crap the day after eating grains (I don't for example) and 2) That game animals were plenty during paleolithic times.

Speaking to the game animals, yes there were tons around. Doesn't mean they were easy to hunt, nor that there were tons around during the deep winter. Grains don't run away from you, and you can store them for the lean times.

The paleolithic era may have been an era of plenty, but for how long? As human populations expanded, we arguably hunted several large game animals to extinction, or near extinction. When those food sources were no longer adequate to sustain the population, we obviously had to look elsewhere.

We can argue today that we can manage livestock to some degree, such that our meat sources are sustainable. But how did paleolithic man manage sustainable agriculture, let alone sustainable hunting practices, without curbing population growth? They didn't.

I think the best way to answer this Q is to think about in terms of group competition - i.e., evolution on the level of group selection. The non-grain-eaters must stay in tribes of about 150 or less (see Robin Dunbar on where this number comes from). The grain-eaters, in contrast, are able to establish much larger groups, which also requires the development of more sophisticated forms of social organization (and perhaps the evolution of enhanced social cognition), which in turn will translate into more effective war capabilities. Thus when Group <150 faces the more organized and sophisticated Group >>150, the latter wins, and this keeps happening until the Group <150s are nearly wiped out wherever Group >>150s exist.

I found a pretty good article about the topic The Natural History of Wheat.

I suspect the reason would be directly related to the creation of stationary communities but that is based on my recall of old college classes in ancient societies.

People made whatever food they could in order to survive. Ancient people weren't worried about insulin response and irritable bowel syndrome, they were worried about survival. If they could figure out how to grow and/or harvest grains, they had a stable food supply.

There are a LOT of people in the world that eat grains every day and don't feel bad. I wouldn't assume that, because you feel bad when you eat them, that all human kind has had the same response.

Most cultures only became cultures because of a stable source of food that was based on grains. Wheat, barley, rice, corn, and other grains were all staples of many cultures. There are no examples (that I'm aware of) of hunter-gatherer cultures that developed significant technology, governance, etc. When you think of the great cultures of humankind such as the Babylonians, Mayans, etc. throughout history, they all had grains as a source of food.

Have you seen those 'Ancient Aliens' shows? Bear with me, I'm not a believer in that stuff, but it's SO interesting. (I do totally love alien movies, though.) According to the people on that show, these aliens basically started civilization for whatever reason, and there are all these carvings and murals and stuff showing "aliens" giving humans technology and knowledge, etc. What if aliens gave us this knowledge of agriculture? Or, I don't know... sometime in the future, we come up with a way to travel back in time, and we introduce it to our ancestors? They're not "wands of power" the aliens are handing to the humans, but stalks of wheat. Way unlikely, but fun to think about at any rate.

I'm afraid I'm going to get down-voted for my response, but it's just something I've sort of daydreamed about. :/

And the grains back then were different from what we've got going on today, and people probably soaked and sprouted those grains to make them edible, so they didn't have as big of an irritating effect that grains do now. And no, eating a piece of wonderbread isn't going to outright get you HIGH, but it does have a sort of calming, relieving effect. Ever notice how much in a daze people are when they're eating that crap? Or do you remember when you zoned out eating something with gluten in it? It's obviously not like heroin; it's very, very subtle..but it's still chemically addicting, not just psychologically because you 'know you shouldn't eat it'. There are many reasons why we FIRST started eating grains, and one reason we KEPT eating grains is because they're addicting.

Also worth reading Against The Grain by Richard Manning.

They needed something to eat while creating (and then worshipping at) their ancient temples:

http://ngm.nationalgeographic.com/2011/06/gobekli-tepe/mann-text/1

Besides the advantage to storage - it's cheap and filling (at least for a little while) - as a former vegetarian - there are many uses that seem to satisfy the protein - even if it's a lie!!! I think as far as feeding the world is concerned - cheap - travels and stores - a seemingly good alternative to trying to feed the world meat!

Climate change is the popular theory: http://citeseerx.ist.psu.edu/viewdoc/download?doi=10.1.1.61.3692&rep=rep1&type=pdf