My answer deals specifically with civilization in ancient Mesopotamia.

Humans were hunters and gatherers until just after 10000 BC, when we see man begin to cultivate crops (mainly cereals) and also the domestication of wild animals. These changes did not occur overnight though, and it was a long time before cultivation of crops and domestication of animals would replace hunting and gathering as the main source of food.

With the adoption of agriculture and animal husbandry the human population began to rise. Humans engaged in agriculture were also forced to settle, and families banded together for protection and greater food production. Most of the settled land would be controlled by extended families. The first settlements in Babylonia were founded around 5000 BC, and are known as Ubaid.

The most significant feature of Ubaid settlements was their use of irrigation for grain production. Irrigation comes with many difficulties, and requires many people. The need for a large, well-organized labor force for clearing canals and keeping the river banks sound would lead to one of the earliest "officials" in human history, the canal inspector. This would shape their social system, as it meant that people would have to live in large units and accept centralized direction of labor.

Another important factor dealing with the creation of social stratification was the fact that some land would be more productive than other land. The families on the best land would become more prosperous. Settlements would grow into villages. And so we would see inequality in wealth and power.

Also, religion was also a major componet of life in ancient Mesopotamia and religious institutions would come to hold much wealth and become very powerful.

The Babylonians, H.W.F. Saggs