Everyone knows that line. If your intellectual formation is like mine, it has approximately the same status as "In the beginning God created the heavens and the earth."
The rabbis, I'm told, used to like to point out that in Hebrew the first letter of "In the beginning..." looks like a box with three sides, and only one opening. This was to convey the message, don't ask what happened before, or what might be happening somewhere else. The only story we care about starts here.
We all have our rabbis. But we keep asking anyway, what comes before? What comes before that first sentence of Capital, what's happening elsewhere? What form does wealth (claims on the good life) take in societies where the capitalist mode of production doesn't prevail? And apart from how it appears, or presents itself, what can we say about what it really is?
I think the biggest problem with how people read Capital is, they don't take the subtitle seriously: It really is a "critique of political economy." There's an overarching irony, the whole thing is written under the sign of the hypothetical. (In general, I think this irony is one of the most important, and hardest, things that students have to learn in any field.) The whole book is written to show that even if everything Ricardo said was true, capitalism would still be an unjust and inhumane (and unstable, though that doesn't come til later) economic system. But that's not the same as saying that everything Ricardo says really is true. In my opinion -- people I respect disagree -- everything Marx says about the Labor Theory of Value is preceded by an implicit "even if..." It shouldn't be interpreted as a set of positive claims about the world.
So what does Marx positively believe? For this, I think we have to turn to the early writings. I know these are deep waters, on which I am innocently paddling about in my little water wings. But in my opinion, the Economic and Philosophic Manuscripts of 1844 are the essential "before" to Capital.
In Capital, exploitation is defined in terms of the share of the product (already quantifiable; the transformation of the infinitely heterogeneous content of human activity into a mass of commodities has already taken place) to which claims accrue as a result of wage labor. But in the Manuscripts, he says
A forcing-up of wages (disregarding all other difficulties, including the fact that it would only be by force, too, that the higher wages, being an anomaly, could be maintained) would ... be nothing but better payment for the slave, and would not conquer either for the worker or for labour their human status and dignity.Or equivalently, “The alienation of the product of labour merely summarizes the alienation in the work activity itself.” What's important is exhausting one's creative powers on alien ends. How many channels you have on tv afterward doesn't matter.
Alienated labor means (to take various of Marx's definitions): “the work is external to the worker”; the worker "does not fulfill himself in his work”; the worker “does not develop freely his mental and physical energies”; “work is not voluntary, but imposed, forced labour”; work “is not the satisfaction of a need, but a means for satisfying other needs”; the worker “does not belong to himself but to another person.” This is the non-quantifiable fact about life under life under capitalism for which questions about the distribution of commodities are a stand-in. Everything that happens in Capital, happens after this.