Because the well-being of the community depended upon close observation of natural phenomena, scientific or protoscientific 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 the Semitic city of Akkad. Sargon was the world's first empire-builder, sending his troops as far as Egypt and Ethiopia. He attempted to establish a unified empire and to end the hostilities among the city-states. Sargon's rule introduced a new level of political organization that was characterized by an even more clear-cut separation between religious authority and secular authority. To ensure his supremacy, Sargon created the first conscripted army, a development related to the need to mobilize large numbers of laborers for irrigation and flood-control works. Akkadian strength was boosted by the invention of the composite bow, a new weapon made of strips of wood and horn.
Despite their military prowess, Akkadian hegemony over southern Mesopotamia lasted only 200 years. Sargon's great-grandson was then overthrown by the Guti, a mountain people from the east. The fall of the Akkadians and the subsequent reemergence of Sumer under the king of Ur, who defeated the Guti, ushered in the third phase of Sumerian history. In this final phase, which was characterized by a synthesis of Sumerian and Akkadian cultures, the king of Ur established hegemony over much of Mesopotamia. Sumerian supremacy, however, was on the wane. By 2000 B.C. the combined attacks of the Amorites, a Semitic people from the west, and the Elamites, a Caucasian people from the east, had destroyed the Third Dynasty of Ur. The invaders nevertheless carried on the Sumero-Akkadian cultural legacy.
The Amorites established cities on the Tigris and the Euphrates rivers and made Babylon, a town to the north, their capital. During the time of their sixth ruler, King Hammurabi (1792-1750 B.C.), Babylonian rule encompassed a huge area covering most of the Tigris-Euphrates river valley from Sumer and the Persian Gulf in the south to Assyria in the north. To rule over such a large area, Hammurabi devised an elaborate administrative structure. His greatest achievement, however, was the issuance of a law code designed "to cause justice to prevail in the country, to destroy the wicked and the evil, that the strong may not oppress the weak." The Code of Hammurabi, not the earliest to appear in the Near East but certainly the most complete, dealt with land tenure, rent, the position of women, marriage, divorce, inheritance, contracts, control of public order, administration of justice, wages, and labor conditions.
In Hammurabi's legal code, the civilizing trend begun at Sumer had evolved to a new level of complexity. The sophisticated legal principles contained in the code reflect a highly advanced civilization in which social interaction extended far beyond the confines of kinship. The large number of laws pertaining to commerce reflect a diversified economic base and an extensive trading network. In politics, Hammurabi's code is evidence of a more pronounced separation between religious and secular authority than had existed in ancient Sumer. In addition to Hammurabi's legal code, the Babylonians made other important contributions, notably to the science of astronomy, and they increased the flexibility of cuneiform by developing the pictogram script so that it stood for a syllable rather than an individual word.