The earliest documented inhabitants of southern Mesopotamia (southern Iraq), c 3500 BC, considered the world's first civilization. Located between Babylon and the head of the Persian Gulf, these people spoke a language unrelated to any other known language. Formed originally by the need for irrigation agriculture, they created a social and political organization, their own art, literature, and religious observances and greatly influenced neighboring cultures. Cities appeared, such as Eridu, Lagash, Uruk, and Ur, with craft specialization and accumulation of wealth. Most important was the invention of writing. The cuneiform script developed for writing Sumerian can be read. The political unit was the city-state, in which the patron deity, through the priesthood and temple organization, was the major power in all matters. Secular rulers were required only in time of war. The various city-states were united by a common culture and religion, the patron deities such as Enki, Enlil, Nannar, and the rest being members of a single Sumerian pantheon. Sumer was conquered by the Semites of Akkad under Sargon c 2370 BC. The Sumerian culture survived this and later foreign conquests with very little change. Some scholars believe that the Sumerians go back much further and may even have been the first sedentary inhabitants of southern Mesopotamia, from about 5500 BC. The Sumerian language had invariable bisyllabic or monosyllabic roots, around which prefixes or suffixes, also invariable, were arranged to express grammatical inflections. The structure of the language must have made it easier to invent writing and, in a second period, the use of syllabic characters. Sumerian overtaken by Babylonian and ceased to be spoken at beginning of 2nd millennium BC, but then became a language used for cultural purposes and retained that function until cuneiform writing itself disappeared in 1st century AD.