Beer, plain and simple. We settled to consume alcoholic beverages. And I'm not being cheeky here. I heard that every early civilization had some form of beer like product. I am thinking that early nomadic and tribes cached food in places, much the aboriginal tribes of Australia. I speculate that enough of these caches, which likely contained grain products, may have gotten wet and fermented. People being the way they are still consumed the mix and found it to be good.. This happened frequently enough the people began to figure out how to do it themselves, and we ended up settling so we could produce it on a larger scale. I don't think agricultural civilization would have survived with out the ability to get drunk.

That's my thoughts anyway. I got no sources.