To continue the value theory discussion in light of Marx's writing in the Grundrisse, I could try and attempt a longer thought with more thorough comments, but I'll keep it short and just say: correct me if I'm wrong:
The fact that labor power can produce more than its value is not due to some magical properties, but boils down to the historically attained level of productivity of labor. If productivity were such that every adult would have to spend all day (minus sleep) to gather enough food just to survive (this is equivalent to the situation where the product of labor is equal to the value of labor power), there would be no surplus product. Capitalism, and class society in general, would be impossible. However, humans must have reached a level of productivity allowing for some surplus product pretty early in their (pre)history. Since value is defined as "congealed", "objectified", "abstract", social (human) labor, Marx's theory of value is a theory about how human labor is regulated in a commodity-producing society. It's a social theory, to put it bluntly, about how people act. Marx saw the apportioning of human labor as the central, ever-present task for all societies. Social labor is what makes the social world go around. So he set about constructing a theory that explains how it works in capitalism. It could be expected that the central concept of that theory would be about... human social labor.
It is (abstract) human social labor that creates value, while the operation of a machine or the activity of an animal doesn’t and/ or can’t. Just one caveat, for good measure, human concrete labor is certainly not the substance of value: it is abstract labor. Abstract labor didn't always exist; there have always been things with the property of being human social labor, some kind of concrete labor. That category is transhistorical and discussed e.g., by Aristotle. Abstract labor is quite different, i.e., a historical formation of concrete labor. The proposition arises within a theory of the different forms of social re-production, or social labor. In the specifically capitalist social (production) relations, just as most any work in any period takes the form of a work, a product, so here the "private", "abstract", latently-social labor instantiated in alienated work trapped in capitalist production relations takes the form of (the product’s) value.
Now, the question is: why that in capitalism human social labor takes the form of value? The general answer is, because of the (historically specific) division of labor typical of general commodity production. Capitalism is premised on atomized, private producers (that don't produce according to a preexisting social plan, nor are they directed by a higher authority. They produce whatever they want, throw it at the market, and then wait whether it will be recognized as socially useful and as part of total social labor (and to what quantitative extent). If it is, this is a signal to go on producing. If it isn't, it's a signal to switch branches or go bankrupt. This is extremely simplified but still understandable, I hope. Whereas in a non-capitalist economy, human labor is directly social (it is apportioned prior to the fact of being set in motion); in capitalism, the private labor only becomes recognized as social through the market. So the apportioning of human social labor in non-capitalist societies is done "ex ante", while in capitalism it's "post festum". The market gives signals and social labor is apportioned based on them.
So, human social labor has always and will always exist -- "the calorie is life's basic unit", etc., etc. In pre-capitalist societies, concrete labor is directly social. This because of how these societies function. There is an authority or a plan or a tradition that apportions human labor. However, human social labor takes the form of value, i.e., of an apparent property of things, in systems based on a private divison of labor. In these systems, concrete labor is not directly social. It is rendered social through exchange. So, for human labor to be producing value, you have to have a particular social division of labor. Other forms of "labor" (operations of machines, activities of animals) can't be value-producing, because they're not human labor. This means that only until the revolution happens, we're stuck with the value theory of labor (which is a much more fitting name for Marx's theory than "labor theory of value").
