The world's earliest settled civilization, which arose in southern Mesopotamia, dating to the mid-sixth millennium B.C.E. By the late fourth millennium, the world's first urban centers appeared there.

Archaeologists believe that the earliest Sumerians were a farming people who inhabited northern Mesopotamia before moving south as early as 5200 B.C.E. The region's arid climate and limited water resources required the development of effective irrigation systems to sustain an agricultural lifestyle. Creating and maintaining such systems required a high degree of social organization that eventually enabled the growth of increasingly larger permanent settlements.

The surplus food generated by these settlements permitted a portion of the population to devote itself to duties other than food production. Ruling and priestly classes emerged to organize and direct labor, and artisans such as pottery makers, weavers, and metalsmiths developed specialized skills to produce luxury and trade goods. By the mid-fifth millennium B.C.E., Sumerians had developed a flourishing trade along the Tigris and Euphrates rivers. Some time before 3000 B.C.E., the wealth generated by trade enabled local rulers to found the first cities based around central temple complexes.

The Sumerian civilization consisted of a dozen city-states located between the Tigris and Euphrates rivers, the largest of which were Ur, Uruk, Lagash, and Kish. A city-state was self-governing city, usually guarded by defensive walls, and the agricultural lands surrounding it. The most important structure in a Sumerian city was the ziggurat, a monumental building atop which was located a temple dedicated to the city's patron god. The patron god of Ur, for example, was the moon god, Nanna. A statue of the god, who was believed to look after the welfare of the city, was located in the temple.

Sumerian society was highly organized and hierarchical. At the top was the king who served as both the head of the government and the commander of the military. Although accepted by the people as the representative of the city's patron god, the king was not considered divine. The king was advised by priests who served the patron god, and by a council of elders. Approximately 10 percent of the city's population worked as merchants or produced goods such as pottery, leather goods, and textiles for local consumption as well as for trade. Ninety percent of the population worked to produce food. Slavery was common, and foreigners captured in warfare worked in a wide range of menial tasks, particularly agriculture and construction.

The Sumerians made some of the most significant contributions to early human civilization. In the fifth century B.C.E., the Sumerians are believed to have invented the wheel, which they used not only for transportation (by the mid-fourth millennium B.C.E.), but also for use in making pottery. Before the invention of the wheel, potters generally created their pieces by coiling strips of wet clay and stacking the coiled strips atop one another. This technique limited the size and shapes of pots that could be produced. The pottery wheel allowed artisans to work with larger lumps of clay that could be rotated on the wheel and shaped with both hands. This enabled them to form larger pots with thinner, yet sturdier, walls in a variety of shapes and sizes.

Around 3100 B.C.E., the Sumerians developed a system of writing called cuneiform, originally used to record commercial transactions. The uses of writing eventually expanded to include business contracts, legal decrees, religious rites and beliefs, and the stories and myths of the people. The most famous of these stories, the Epic of Gilgamesh, is considered the world's first epic poem. Later Mesopotamian civilizations, including the Babylonian and Assyrians, adopted the use of cuneiform, which served as the region's principal writing system for nearly 2,000 years.

Although the Sumerian city-states belonged to a single civilization, there was little permanent cooperation among them. Control of scarce resources, territorial expansion, and control over trade routes were sources of frequent disputes and conflicts. This constant fighting, and the inability of the city-states to share resources or unite for common defense, made Sumer vulnerable to attack from its neighbors. The Akkadians, who dominated the region northwest of Sumer under their king Sargon (r. ca. 2334–ca. 2279 B.C.E.) of Akkad, conquered Sumer around 2330 B.C.E. Sargon united the Sumerian city-states with his kingdom in northern Mesopotamia, forming an empire that extended from the Persian Gulf to parts of present-day Turkey.

Around 2150 B.C.E., the Akkadian Empire was invaded by the Gutians, a nomadic tribe from the Zagros Mountains in what is now Iran. This led to a period of instability in the region, as the Gutians were not equipped to rule over a complex society such as had developed in Sumer. A new Sumerian dynasty arose when Ur-Nammu (r. ca. 2065–ca. 2047 B.C.E.) restored Sumerian control over the region. Fragments of clay tablets dating to his reign contain remnants of the world's oldest known law code. Ur-Nammu was succeeded by his son, Shulgi (r. 2047–1999 B.C.E.), who greatly expanded the network of roads that facilitated trade throughout the empire.

The remaining Sumerian kings were unremarkable rulers, and in 1957 B.C.E., the Elamites, who lived east of Sumer, invaded and took temporary control of the region. By 1900 B.C.E., the Amorites, who lived west of the Euphrates, had moved into Mesopotamia and emerged as the region's dominant group. Because they established a capital in the city of Babylon, they are known as the Babylonians. Although the rise of the Babylonian Empire marked the end of Sumerian civilization, the Babylonians adopted key aspects of Sumerian culture. As a result, Sumerian religion and social customs remained powerful influences throughout Mesopotamia for many centuries.

The Royal Standard of Ur, dating to about 2750 B.C.E., is a mosaic depicting aspects of society in ancient Sumer. The panel, unearthed from royal graves in the southern Mesopotamian city of Ur, contains rows of figures on both sides. One side (shown here) contains images of war while the other features images of peacetime. (The Bridgeman Art Library/Getty Images)