Mesopotamia, at 7,000 BC, even at 5,000 BC, was mostly a wide open place. But as rulership grew and spread, life there was progressively choked. By 3,000 BC, the entire area was controlled by centralized power… by a vast network of kings, overseen by a "king of kings."
More than 100 factors of daily life were said to be ordained by the gods, and enforced by their representatives. And so life became stratified, which means that people were held in place: farmers and their children remained farmers, temple officials and their children remained temple officials, and so on.
In other words, the more rulership spread, the more the world of Mesopotamia was turned into a hierarchy, with people from the highest layers (kings, priests and nobles) telling people at the lower layers what they were expected to do, and punishing those who didn't comply.
Sumerian cities were stratified into four primary layers:
Nobles. At the top would be the temple overseer (called an en), as well as a ruler (called an ensi). Over time, the balance between temple and palace – between priest and king, en and ensi – varied. Also near the top were wealthy families, who owned large tracts of land. (A good deal of land in the city center was owned by the temple, and still other parts by the ruler.) People from these families met in assemblies to decide (or more likely to recommend) certain collective actions.
Clients. Next were the clients: people who were important to those in the top layer. These were temple administrators, important craftsmen and the family members of the nobility.