David Graeber has a really excellent post on money and the Myth of Barter -- the view that primitive trade is a barter system and that money develops as a way to facilitate such bartering. Exchanges and debts are pretty general features of the human condition, of course, but as anthropologists and historians have looked at the evidence, it has become increasingly clear that this story is false. Part of the problem is a lack of imagination -- the notion that if an exchange system is not money-based, it is necessarily a barter system, when in fact the ways and means of exchange are extraordinarily diverse (barter is widespread, but you can have gift economies, for instance, that work on completely different principles, and you can have economies largely based on highly structured ritualized transfers rather than any bartering). And part of the problem is that barter systems, where they exist, seem usually to presuppose money as a means of equalizing barter; that is, it would be more plausible -- although still perhaps not quite right -- to say that barter is a result of money than that money is a result of barter. What would really be more accurate to say is that, while occasional barter occurs spontaneously, bartering is not the crude and primitive economic interaction it is often treated as being, but requires, in order to be systematic, rather sophisticated conventions already to be in place. As Graeber notes, systematic barter typically requires either the development of traditional equivalences based purely on consistencies in trade (which are unlikely ever to require the development of a standardized money) or the pre-existence of at least a crude money-based accounting. Systematic barter is neither crude nor simple; but people keep thinking of it as if it were, ignoring the whole pack of assumptions involved in that. It's likely there are other factors involved in the endurance of the Myth of Barter.
Graeber's reply is a response to criticisms of his interview, which is also worth reading. You do have to be a little careful reading him, because he has a sense of humor that sometimes is a bit subtle.