Marx (in his book Capital) explains that capitalism could only come about once a population was market dependent. That means that people need to buy and sell things in order to survive.

In pre-capitalist societies (such as feudal Europe), people produced pretty much everything they needed themselves. They grew a little bit of grain, a little bit of barley, and probably raised some cows and sheep for some milk and wool. There was very little surplus, any extra was forcibly expropriated by the local ruler.

Marx tells us that external political factors begin to change this situation. In England at least, enclosing of land by feudal rulers forced most people off their land. They were then hired to farm the land by the once feudal ruler and now capitalist landowner. The capitalist landowners paid these workers a wage which they then used to buy food, clothes, and anything else that they needed.

In Marxist terms, the landowner now owned the means of production and hired landless workers to farm. Everybody in this scenario is now market dependent, they don't produce things for themselves, they are forced to go to a market to purchase the things they need to survive.

My source for this is Part Eight of Marx's Capital, entitled Primitive Accumulation.

If you have any followup questions feel free to ask.