Beginning around 3500 B.C.,an urban revolution occurred independently in multiple centers around the world. The same remarkable patterns of settlements coalescing into centralized kingdoms based on intensified agriculture occurred at least six times at six different sites around the globe during the Neolithio period; in Mesopotamia after 3500 B.C., in Egypt after 3400 B.C.,in the Indus River valley after 2500 B.C.,along the Yellow River in China after 1800 B.C.in Central America at about 500 B.C.and in South America after 300 B. The origin and development of these civilizations were essentially independent and not the result of diffusion (or spreading) from a single center,and hence they are known as the pristine civilizations.

Why did civilizations arise independently and repeatedly on a. worldwide scale after the fourth millennium B.C. in those particular sites? Several explanations have been proposed. The precise processes involved in the leap to civilization are research questions actively debated by archaeologists and anthropologists, but many scholars emphasize the importance of hydrology (the study of water on Earth and in the atmosphere) and the physical environment, and they recognize that intensified agriculture, made possible by large-scale hydraulic-engineering (irrigation) projects, was a key element in the formation of large, highly centralized bureaucratic states, The fact alone that pristine civilizations arose in hydrologically distressed regions-that is, where too little or too much water required hydraulic engineering for the successful practice of intensified agriculture-gives credence to what is called the hydraulic hypothesis.

The hydraulic hypothesis links the rise of civilization to the development of technology for large-scale hydraulic systems. Under a hot, semitropical sun, irrigation agriculture is extraordinarily productive, and a yield that can support large populations becomes possible. Rivers carrying sediment provide water for irrigation and ,especially when controlled artificially, they enrich the soils around them. Irrigation agriculture and flood control required hydraulic-engineering systems and some level of communal action to build and maintain them and to distribute water when and where needed: marshes had to be drained; dams, dikes, canals, terraces, and embankments had to be built; and ditches had to be kept free of debris. Water disputes had to be settled by some authority, and grain surpluses had to be stored, guarded, and redistributed. The interacting effects of the geographical setting and the techniques of hydraulic agriculture reinforced trends toward an authoritarian state.

That model described above admirably fits what happened with the first human civilization arising on the floodplain between the Tigris and Euphrates Rivers in what ls present-day lraq.This was ancient Mesopotamia, the land "between the rivers." By 4000 B.C.Neolithic villages filled the Mesopotamian plain.Local authorities drained marshes and installed extensive irrigation works on the flood plain Great walled cities arose after 3500 B.C., and the Sumerian dynasty developed fully by 2500 B.C..

The notion of "environmental circumscription" provides a key explanation for this rise in urban development. Environmental circumscription occurs when an area of land is surrounded by a less productive area such as mountains,desert, or sea. civilizations arose in prehistoric river valleys and floodplains that were environmentally restricted agricultural zones beyond which intensive farming was impossible or impractical. In these constricted habitats, like the Nile River valley, expanding Neolithic populations soon pressed against the limits imposed by the surrounding desert, waterfalls, and sea, leading to pressures to intensify food production. Warfare became chronic and developed beyond mere raiding (attack and retreat)to involve conquest and subjugation since, in a habitat already filled, the defeated peoples could no longer go off and form a new agricultural community. Whereas previously,defeated groups could generally move on to a new locale, in environmentally restricted areas such as the Nile River valley, agricultural communities had nowhere to go.

Victors not only took over land and smaller irrigation works but subjugated and dominated conquered groups,sparing their lives in return for their labor as slaves and peasants in maintaining systems of intensified farming. Once this process started,the historical momentum favoring unifying and centralizing forces was irreversible.Neolithic communities thus became increasingly stratified, culminating in a dominant elite in command of ar agricultural underclass as regional powers subsumed local ones. Time and again civilization and the state emerged wherever these environmental and demographic conditions occurred.