<P> Uruk, one of Sumer's largest cities, has been estimated to have had a population of 50,000 - 80,000 at its height; given the other cities in Sumer, and the large agricultural population, a rough estimate for Sumer's population might be 0.8 million to 1.5 million . The world population at this time has been estimated at about 27 million . </P> <P> The Sumerians spoke a language isolate, but a number of linguists have claimed to be able to detect a substrate language of unknown classification beneath Sumerian because names of some of Sumer's major cities are not Sumerian, revealing influences of earlier inhabitants . However, the archaeological record shows clear uninterrupted cultural continuity from the time of the early Ubaid period (5300--4700 BC C - 14) settlements in southern Mesopotamia . The Sumerian people who settled here farmed the lands in this region that were made fertile by silt deposited by the Tigris and the Euphrates . </P> <P> Some archaeologists have speculated that the original speakers of ancient Sumerian may have been farmers, who moved down from the north of Mesopotamia after perfecting irrigation agriculture there . The Ubaid period pottery of southern Mesopotamia has been connected via Choga Mami transitional ware to the pottery of the Samarra period culture (c. 5700--4900 BC C - 14) 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 eight levels yielded pre-Ubaid pottery resembling Samarran ware . According to this theory, farming peoples spread down into southern Mesopotamia because they had developed a temple - centered social organization for mobilizing labor and technology for water control, enabling them to survive and prosper in a difficult environment . </P> <P> Others have suggested a continuity of Sumerians, from the indigenous hunter - fisherfolk traditions, associated with the bifacial assemblages found on the Arabian littoral . Juris Zarins believes the Sumerians may have been the people living in the Persian Gulf region before it flooded at the end of the last Ice Age . </P>

Where was sumer located and why was that important