check out debt the first 5000 years (linked elsewhere in this thread) - its really really good, and it goes into some depth about the various origins of monetary-type systems and currency money, but also how currency systems arose and then fell out of usage many times. in much of medieval europe, actual currency money was in very limited use except among the mercantile classes, with peasants largely taxed in kind or by corvee and having little or no interaction with formal money (this is of course a gross oversimplification). demanding taxes be paid in the currency of the realm has generally been used as a way of forcing people into interacting with markets (either by selling their produce or selling their labour) in order to obtain the currency they need to pay to their lords