- Four important early civilizations developed along major river valleys by 2000 BC, including in Mesopotamia between the Tigris and Euphrates Rivers.
- In Mesopotamia, the Sumerians established the first cities and organized into independent city-states for governance as populations grew, with priests initially in control and later military leaders becoming monarchs.
- Sumerian civilization was characterized by advanced cities, specialized workers, complex institutions, record keeping, and technology, establishing a pattern that influenced later peoples in the region.