Term "Sumerian" is an exonym (a name given by another group of people), first applied by the Akkadians. The Sumerians described themselves as "the black-headed people" (sag-gi-ga) and called their land ki-en-gir, "place of the civilized lords". The Akkadian word Shumer possibly represents this name in dialect.
The Sumerians, with a language, culture, and, perhaps, appearance different from their Semitic neighbors and successors were at one time believed to have been invaders, but the archaeological record shows cultural continuity from the time of the early Ubaid period (5200-4500 BC C-14, 6090-5429 calBC) settlements in southern Mesopotamia.
The challenge for any population attempting to dwell in Iraq's arid southern floodplain was to master the Tigris and Euphrates river waters for year-round agriculture and drinking water. In fact, the Sumerian language is replete with terms for canals, dikes, and reservoirs, indicating that Sumerian speakers were farmers who moved down from the north after perfecting irrigation agriculture there.
The Ubaid pottery of southern Mesopotamia has been connected via 'Choga Mami Transitional' ware to the pottery of the Samarra period culture (5700-4900 BC C-14, 6640-5816 calBC) in the north, who were the first to practice a primitive form of irrigation agriculture along the middle Tigris river and its tributaries.
The connection is most clearly seen at Tell Awayli (Oueilli/Oueili) near Larsa, excavated by the French in the 1980s, where 8 levels yielded pre-Ubaid pottery with affinities to Samarran ware.
Sumerian speakers spread down into southern Mesopotamia because they had developed a social organization and a technology that enabled them, through their control of the water, to survive and prosper in a difficult environment where, other than a possible indigenous hunter-gatherer population in the marshlands at the head of the Arabo-Persian Gulf and seasonal pastoralists, they had no competition.
A distinctive style of painted pottery spread throughout Mesopotamia in the Ubaid period, when the ancient Sumerian cult-center of Eridu was gradually surpassed in size by the nearby city of Uruk. The archaeological transition from the Ubaid period to the Uruk period is marked by a gradual shift from painted pottery domestically-produced on a slow wheel, to a great variety of unpainted pottery mass-produced by specialists on a fast wheel.
The date of this transition, from Ubaid 4 to Early Uruk, is in dispute, but calibrated radiocarbon dates from Tell Awayli would place it as early as 4500 BC.By the time of the Uruk period (4500-3100 BC calibrated), the volume of trade goods being inexpensively transported along the canals and rivers of southern Mesopotamia facilitated the rise of many large temple-centered cities where centralized administrations could afford to employ specialized workers. It is fairly certain that it was during the Uruk period that Sumerian cities began to make use of slave labor, and there is ample evidence for captured slaves as workers in the earliest texts.
Artifacts, and even colonies of this Uruk civilization have been found over a wide area - from the Mediterranean sea in the west, to the Taurus Mountains in Turkey, and as far east as Central Iran.
The Uruk period civilization, exported by Sumerian traders and colonists, had a stimulating and influential effect on surrounding peoples, who gradually evolved their own comparable, competing economies.
The cities of Sumer could not maintain remote, long-distance colonies purely by military force; the domestic horse did not appear in Sumer until the Ur III period - one thousand years after the Uruk period ended. The end of the Uruk period coincided with a dry period from 3200-2900 BC that marked the end of a long wetter, warmer climate period from ca. 9,000 to 5,000 years B.P. called the Holocene climatic optimum.
When the historical record opens, the Sumerians seem to be limited to southern Mesopotamia, although very early rulers such as Lugal-Anne-Mundu are indeed recorded as expanding to neighboring areas as far as the Mediterranean, Taurus and Zagros, and not long after legendary figures like Enmerkar and Gilgamesh, who are associated in mythology with the historical transfer of culture from Eridu to Uruk, were supposed to have reigned.
The term 'Sumerian' applies to speakers of the Sumerian language. The Sumerian language is generally regarded as a language isolate in linguistics because it belongs to no known language family; Akkadian belongs to the Afro-Asiatic languages.