I'm not convinced by "complex hierarchies and taxation schemes". It sounds just as plausible that grain was one of the first really tradeable forms of food, so people became able to trade non-food-related skills for food, with people from further away. Your ability to do this is badly limited if you're using wet foods like potatoes, because you are only able to trade with people fairly nearby before the potatoes start sprouting; but with grain you can send your son off to the market three hours' wagon-ride away with a month's worth of crop, for instance.