Let’s start with what money is used for. Modern economists typically define it by the three roles it plays in an economy: 


It’s a store of value, meaning that money allows you to defer consumption until a later date. 


It’s a unit of account, meaning that it allows you to assign a value to different goods without having to compare them. So instead of saying that a Rolex watch is worth six cows, you can just say it (or the cows) cost $10 000.


And it’s a medium of exchange—an easy and efficient way for you and me and others to trade goods and services with one another.


All of these roles have to do with buying and selling, and that’s how the modern world thinks of money—so much so that it seems peculiar to conceive of money in any other way. 


Yet in tribal and other “primitive” economies, money served a very different purpose—less a store of value or medium of exchange, much more a social lubricant. As the anthropologist David Graeber puts it in his recent book Debt: The First 5000 Years (Melville House, 2011), money in those societies was a way “to arrange marriages, establish the paternity of children, head off feuds, console mourners at funerals, seek forgiveness in the case of crimes, negotiate treaties, acquire followers.” Money, then, was not for buying and selling stuff but for helping to define the structure of social relations. 


How, then, did money become the basis of trade? By the time money makes its first appearance in written records, in Mesopotamia during the third millennium B.C.E., that society already had a sophisticated financial structure in place, and merchants were using silver as a standard of value to balance their accounts. But cash was still not widely used.


It’s really in the seventh century B.C.E., when the small kingdom of Lydia introduced the world’s first standardized metal coins, that you start to see money being used in a recognizable way. Located in what is now Turkey, Lydia sat on the cusp between the Mediterranean and the Near East, and commerce with foreign travelers was common. And that, it turns out, is just the kind of situation in which money is quite useful. 


To understand why, imagine doing a trade in the absence of money—that is, through barter. (Let’s leave aside the fact that no society has ever relied solely or even largely on barter; it’s still an instructive concept.) The chief problem with barter is what economist William Stanley Jevons called the “double coincidence of wants.” Say you have a bunch of bananas and would like a pair of shoes; it’s not enough to find someone who has some shoes or someone who wants some bananas. To make the trade, you need to find someone who has shoes he’s willing to trade and wants bananas. That’s a tough task. 


With a common currency, though, the task becomes easy: You just sell your bananas to someone in exchange for money, with which you then buy shoes from someone else. And if, as in Lydia, you have foreigners from whom you’d like to buy or to whom you’d like to sell, having a common medium of exchange is obviously valuable. That is, money is especially useful when dealing with people you don’t know and may never see again.