Actually, human society hasn't "shifted towards a system where people earned for themselves and kept what they earned". Throughout history there have been motions back and forth towards a amore socialist or a more capitalist system. In fact, I'm not sure you can say that we have such a system in any of the Western countries, right now. People don't just work for themselves: they pool a large chunk of their resources together to build common infrastructure necessary for business.

There was one time, however, when human society shifted from a communal hunter-gatherer society to one having private property, and that was the neolithic revolution. It is pretty much universally agreed that it made most people worse off. They had to work more (you're right about that; hunters-gatherers needed to work only 20 hours a week), got more sick, were malnourished, had to give birth to more children, and lived less (until modern technology, that is). It's unclear why this most important revolution in human history happened, but some theories suggest that it was brought about by the only people who benefitted from it: those who quickly became rich.