The document discusses the early river valley civilizations that developed around 3500 BC, including those in Mesopotamia, Egypt, the Indus Valley, and China. It notes that by 2000 BC, four important civilizations were flourishing in major river valleys and helped sustain agriculture. The Sumerians in Mesopotamia created some of the first cities and organized into independent city-states between rivers like the Tigris and Euphrates, developing advanced societies with specialized workers, complex institutions, record keeping, and technology.