Sumerian civilization coalesces in the subsequent Uruk period (4000 to 3100 BC). Named after the Sumerian city of Uruk, this period saw the emergence of urban life in Mesopotamia and, during its later phase, the gradual emergence of the cuneiform script. Proto-writing in the region dates to around 3500 BC, with the earliest texts dating to 3300 BC; early cuneiform writing emerged in 3000 BC. It was also during this period that pottery painting declined as copper started to become popular, along with cylinder seals. 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. Uruk trade networks started to expand to other parts of Mesopotamia and as far as North Caucasus, and strong signs of governmental organization and social stratification began to emerge leading to the Early Dynastic Period (c. 2900 BC). The Jemdet Nasr period, which is generally dated from 3100 to 2900 BC and succeeds the Uruk period, is known as one of the formative stages in the development of the cuneiform script. The oldest clay tablets come from Uruk and date to the late fourth millennium BC, slightly earlier than the Jemdet Nasr Period. By the time of the Jemdet Nasr Period, the script had already undergone a number of significant changes. It originally consisted of pictographs, but by the time of the Jemdet Nasr Period it was already adopting simpler and more abstract designs. It is also during this period that the script acquired its iconic wedge-shaped appearance. At the end of the Jemdet Nasr period there was a major archaeologically attested river flood in Shuruppak and other parts of Mesopotamia. Polychrome pottery from a destruction level below the flood deposit has been dated to immediately before the Early Dynastic Period around 2900 BC.