I actually had this thought about a week ago, mentally kind of noted it down as a good blog post topic on a day when I have nothing to talk about, and took a photo, knowing eventually I'd stall and need a topic. Today's the day. Here's your photo.
You're looking at it and thinking, 'is that PEANUT BUTTER?' and you're right, it is. Peanut butter on whole wheat toast. Looks like a snack, right? Sure. It's also what built civilization as we know it.
Waaaaay back in the neolithic, we started farming. (We as the human race, of course.) Mostly I suspect that we started off with hard-to-find plants that were hard to gather, and THEN moved on to food crops. But when we did start cultivating food crops, we went with whatever was easiest to grow and most nutritious (though I'm sure that's not how the neolithic farmers would have phrased it, but they understood the concepts). Yield per acre was also a big factor.
Inevitably, no matter where it was in the world, the human race settled on a combination of two families of plants: Grasses and beans. (Poaceae/graminae and fabaceae/leguminosae, if you wanna be a botanist about it.) Always. Every last one of the early civilizations used a combination of grain and beans. In the Middle East, it was wheat and barley for the grain and peas, beans, and chick peas for the beans. China, it was rice and millet for grain and soy beans for the beans. Even central America did it; corn and beans. Sub-Saharan Africa was a latecomer, but used millet and peanuts. Europe used oats, barley, and beans (plus grains and beans imported from the Middle East).
The reason this is the default for agriculture is easy. The two, combined, create a 'full protein', or the whole compliment of amino acids that the body needs to function. Some anthropologists treat the grain and beans combo as interchangeable, but the truth is, not all grains and beans are created equal, and some combinations are better than others for that 'full protein' result. (I once did a research project on this, with charts of amino acids and the works.) According to the research I've done, the best combination is wheat and chick peas. So eat all the whole-wheat pita and hummus that you can hold.
My particular snack of choice has a typically American history. The wheat came with the European settlers, and they got it, originally, in trade all the way from the Middle East (a long, long time ago). The peanuts came with the slaves when millions were stolen from their homes in Africa and brought here. The invention of peanut butter is a hotly debated subject, but most historians these days agree it was not invented by George Washington Carver (though he invented all kinds of other crazy uses for peanuts, including plastics and drugs). It seems that everyone, sooner or later, had the idea to grind up peanuts and use them as a spread or sauce.
Without these 'full proteins' in the diet, mankind would never have had the get-go to build cities, specialize in trades, and generally create what we know as modern civilization.
Not bad for a peanut butter sandwich, huh?