Most of this has to do with complex agriculture. Once people began farming and domesticating animals for food, not every member of society had to be devoted to being a hunter/gatherer, as was previously the case. Once there was a large surplus of food, farmers and herders could feed the population without much need of other assistance, leaving other members of society free to become writers, artisans, merchants, or governors. Once there was a government and currency in place, the government could use taxes to centralize wealth and create a military that would also strengthen the state.
