David Graeber thinks it all began with slavery:
In most times and places, slavery is seen as a consequence of war. Sometimes most slaves actually are war captives, sometimes they are not, but almost invariably, war is seen as the foundation and justification of the institution. If you surrender in war, what you surrender is your life; your conqueror has the right to kill you, and often will. If he chooses not to, you literally owe your life to him; a debt conceived as absolute, infinite, irredeemable. He can in principle extract anything he wants, and all debts – obligations – you may owe to others (your friends, family, former political allegiances), or that others owe you, are seen as being absolutely negated. Your debt to your owner is all that now exists.
He connects this thought to various early societies:
A Babylonian peasant might have paid a handy sum in silver to his wife’s parents to officialise the marriage, but he in no sense owned her. He certainly couldn’t buy or sell the mother of his children. But all that would change if he took out a loan. Were he to default, his creditors could first remove his sheep and furniture, then his house, fields and orchards, and finally take his wife, children, and even himself as debt peons until the matter was settled (which, as his resources vanished, of course became increasingly difficult to do). Debt was the hinge that made it possible to imagine money in anything like the modern sense, and therefore, also, to produce what we like to call the market: an arena where anything can be bought and sold, because all objects are (like slaves) disembedded from their former social relations and exist only in relation to money.
(Hat tip: Kottke)