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 fast wheels.
By the time of the Uruk period (ca. 4100–2900 BC calibrated), the volume of trade goods transported along the canals and rivers of southern Mesopotamia facilitated the rise of many large, stratified, temple-centered cities (with populations of over 10,000 people) where centralized administrations employed specialized workers. It is fairly certain that it was during the Uruk period that Sumerian cities began to make use of slave labor captured from the hill country, 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 Taurus Mountains in Turkey, to the Mediterranean Sea in the west, and as far east as Central Iran.
The Uruk period civilization, exported by Sumerian traders and colonists (like that found at Tell Brak), had an effect on all surrounding peoples, who gradually evolved their own comparable, competing economies and cultures. The cities of Sumer could not maintain remote, long-distance colonies by military force.
Sumerian cities during the Uruk period were probably theocratic and were most likely headed by a priest-king (ensi), assisted by a council of elders, including both men and women. It is quite possible that the later Sumerian pantheon was modeled upon this political structure. There was little evidence of institutionalized violence or professional soldiers during the Uruk period, and towns were generally unwalled. During this period Uruk became the most urbanised city in the world, surpassing for the first time 50,000 inhabitants.
The ancient Sumerian king list includes the early dynasties of several prominent cities from this period. The first set of names on the list is of kings said to have reigned before a major flood occurred. These early names may be fictional, and include some legendary and mythological figures, such as Alulim and Dumizid.
The end of the Uruk period coincided with the Piora oscillation, a dry period from c. 3200–2900 BC that marked the end of a long wetter, warmer climate period from about 9,000 to 5,000 years ago, called the Holocene climatic optimum.