With the rising numbers of the human population after having steady lifestyles, many people come together (those who think pretty much the same) and lived in the same area. These soon formed civilizations like Mesopotamia and the such. They found a common language, and built cities, forts, and towns, while growing and harvesting crops around it. Soon, when the city is large enough, the citizens may start to reach out to other cities to make pacts and NAA (non-aggression-pact).
Now, after years, early civilizations may cooperate by trading resources that each have (that the other doesn't), for example, trading fish for "better" lumber, or trading metal for livestocks. As the trade increases, so do their co-operations, and the more each city cooperates with others, the more products they have. Before, they would have to work with what they can grow, cut-down, and find, but later on, with more resources that they can access, they can build much more advanced buildings, tools, etc. This is how a civilization thrives.
hope this helps
