The civilized life that emerged at Sumer was shaped by two conflicting factors: the unpredictability of the Tigris and Euphrates rivers, which at any time could unleash devastating floods that wiped out entire peoples, and the extreme fecundity of the river valleys, caused by centuries-old deposits of soil. Thus, while the river valleys of southern Mesopotamia attracted migrations of neighboring peoples and made possible, for the first time in history, the growing of surplus food, the volatility of the rivers necessitated a form of collective management to protect the marshy, low-lying land from flooding. As surplus production increased and as collective management became more advanced, a process of urbanization evolved and Sumerian civilization took root.

Sumer is the ancient name for southern Mesopotamia. The Sumerians were highly innovative people who responded creatively to the challenges of the changeable Tigris and Euphrates rivers. Many of the great Sumerian legacies, such as writing, irrigation, the wheel, astronomy, and literature, can be seen as adaptive responses to the great rivers.
The Sumerians were the first people known to have devised a scheme of written representation as a means of communication. From pictograms the Sumerians gradually created cuneiform writing. Through writing, the Sumerians were able to pass on complex agricultural techniques to successive generations; this led to marked improvements in agricultural production.

Another important Sumerian legacy was the recording of literature. The most famous Sumerian epic and the one that has survived in the most nearly complete form is the epic of Gilgamesh. The story of Gilgamesh, who actually was king of the city-state of Uruk in approximately 2700 B.C., is a moving story of the ruler's deep sorrow at the death of his friend and of his consequent search for immortality. Other central themes of the story are a devastating flood and the nature of human existence. With its complex abstractions and emotional expressions, the epic of Gilgamesh reflects the intellectual sophistication of the Sumerians, and it has served as the prototype for all Near Eastern inundation stories, including the Old Testament story of Noah and the flood.

The precariousness of existence in southern Mesopotamia also led to a highly developed sense of religion. Cult centers such as Eridu, dating back to 5000 B.C., served as important centers of pilgrimage and devotion even before the rise of Sumer. Many of the most important Mesopotamian cities emerged in areas surrounding the pre-Sumerian cult centers, thus reinforcing the close relationship between religion and government.

The Sumerians were pantheistic; their gods more or less personified local elements and natural forces. In exchange for sacrifice and adherence to an elaborate ritual, the gods of ancient Sumer were to provide the individual with security and prosperity. A powerful priesthood emerged to oversee ritual practices and to intervene with the gods. Sumerian religious beliefs also had important political aspects. Decisions relating to land rentals, agricultural questions, trade, commercial relations, and war were determined by the priesthood, because all property belonged to the gods. The priests ruled from their temples, built in the shape of a ziggurat with outside staircases that tapered toward a shrine at the top.
Because the well-being of the community depended upon close observation of natural phenomena, basic scientific activities occupied much of the priests' time. For example, the Sumerians believed that each of the gods was represented by a number. The number sixty, sacred to the god An, was their basic unit of calculation. The minutes of an hour and the notational degrees of a circle were Sumerian concepts. The highly developed agricultural system and the refined irrigation and water-control systems that enabled Sumer to achieve surplus production also led to the growth of large cities. The most important city-states were Uruk, Eridu, Kish, Lagash, Agade, Akshak, Larsa, and Ur (birthplace of the prophet Abraham). The emergence of urban life led to further technological advances. Lacking stone, the Sumerians made marked improvements in brick technology, making possible the construction of very large buildings such as the famous ziggurat of Ur. Sumer also pioneered advances in warfare technology. By the middle of the third millennium B.C., the Sumerians had developed the wheeled chariot. At approximately the same time, the Sumerians discovered that tin and copper when smelted together produced bronze--a new, more durable, and much harder metal. The wheeled chariot and bronze weapons became increasingly important as the Sumerians developed the institution of kingship and as individual city-states began to vie for supremacy.

Historians generally divide Sumerian history into three stages. In the first stage, which extended roughly from 3360 B.C. to 2400 B.C., the most important political development was the emergence of kings who, unlike the first priestly rulers, exercised distinct political rather than religious authority. Another important feature of this period was the emergence of warring Sumerian city-states, which fought for control of the river valleys in lower Mesopotamia. During the second phase, which lasted from 2400 B.C. to 2200 B.C., Sumer was conquered in approximately 2334 B.C. by Sargon I, king of Akkad.