History of the world
Whether in prehistoric or historic times, people always needed to be near reliable sources of potable water. Settlements developed on river banks as early as 3000 BCE in Mesopotamia, on the banks of Egypt's Nile River, in the Indus River valley, and along China's rivers. As farming developed, grain agriculture became more sophisticated and prompted a division of labour to store food between growing seasons. Labour divisions led to the rise of a leisured upper class and the development of cities, which provided the foundation for civilization. The growing complexity of human societies necessitated systems of accounting and writing.
Ancient history
Mesopotamia is the site of some of the earliest known civilizations in the world. Early settlement of the alluvial plain lasted from the Ubaid period (late 6th millennium BC) through the Uruk period (4th millennium BC) and the Dynastic periods (3rd millennium BC) until the rise of Babylon in the early 2nd millennium BC. The surplus of storable foodstuffs created by this economy allowed the population to settle in one place instead of migrating after crops and herds. It also allowed for a much greater population density, and in turn required an extensive labor force and division of labor. This organization led to the necessity of record keeping and the development of writing (c. 3500 BC).