Human labor power as a commodity is a defining characteristic of capitalist relations of production. In precapitalist societies, there was exchange of commodities "on the periphery" , but most labor power was not a commodity. Also, in capitalism, the human labor power commodity has a unique characteristic that no other commodity has. It is the only source of new value ( exchange-value !). The labor-power commodity can add more exchange-value in producing commodities than the value of the commodities that went into reproducing it.
A women's liberationist might point out that there is no magic about this, although there is some Mrs.tery. The labor power is produced by the domestic labor power give in the home. "Domestic" or reproductive labor is the only source of the only source of new value. This logic is the basis of a unified Marxist and feminist theory of production and reproduction.
CB
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Surplus Value
At a certain stage in the development of commodity production money becomes transformed into capital. The formula of commodity circulation was C-M-C (commodity-money-commodity)-i.e., the sale of one commodity for the purpose of buying another. The general formula of capital, on the contrary, is M-C-M-i.e., the purchase for the purpose of selling (at a profit). The increase over the original value of the money that is put into circulation is called by Marx surplus value. The fact of this "growth" of money in capitalist circulation is common knowledge. Indeed, it is this "growth" which transforms money into capital, as a special and historically determined social relation of production. Surplus value cannot arise out of commodity circulation, for the latter knows only the exchange of equivalents; neither can it arise out of price increases, for the mutual losses and gains of buyers and sellers would equalize one another, whereas what we have here in not an individual phenomenon but a mass, average and social phenomenon. To obtain surplus value, the owner of money "must ... find... in the market a commodity, whose use-value possesses the peculiar property of being a source of value" [Capital]. <http://www.marxists.org/archive/marx/works/1867-c1/index.htm> -a commodity whose process of consumption is at the same time a process of the creation of value. Such a commodity exists-human labor power. Its consumption is labor, and labor creates value. The owner of money buys labor power at its value, which, like the value of every other commodity, is determined by the socially necessary labor time requisite for its production (i.e., the cost of maintaining the worker and his family). Having bought enough labor power, the owner of money is entitled to use it, that is, to set it to work for a whole day-12 hours, let us say. Yet, in the course of six hours ("necessary" labor time) the worker creates product sufficient to cover the cost of his own maintenance; in the course of the next six hours ("surplus" labor time), he creates "surplus" product, or surplus value, for which the capitalist does not pay. Therefore, from the standpoint of the process of production, two parts must be distinguished in capital: constant capital, which is expended on means of production (machinery, tools, raw materials, etc.), whose value, without any change, is transferred (immediately or part by part) to the finished product; secondly, variable capital, which is expended on labor power. The value of this latter capital is not invariable, but grows in the labor process, creating surplus value. Therefore, to express the degree of capital's exploitation of labor power, surplus must be compared not with the entire capital but only with variable capital. Thus, in the example just given, the rate of surplus value, as Marx calls this ratio, will be 6:6, i.e., 100 per cent.
There were two historical prerequisites for capital to arise: first, the accumulation of certain sums of money in the hands of individuals under conditions of a relatively high level of development of community production in general; secondly, the existence of a worker who is "free" in a double sense: free of all constraint or restriction on the scale of his labor power, and free from the land and all means of production in general, a free and unattached laborer, a "proletarian" , who cannot subsist except by selling his labor power.
http://www.marxists.org/archive/lenin/works/1914/granat/ch03.htm#s1.1