A significant change takes place in Sumerian organization around 2900 BC. Although temples still retained a major role in bureaucracy and economic organization, secular authorities also now emerged. A glimpse of the potential conflict of interests between priesthood and king may be seen in the account in the 'Epic of Gilgamesh' of the bitter feud between Gilgamesh and the goddess Inanna.
The population of Sumer had been steadily increasing and it seems that around 2900 BC it reached the point where conflicts began to develop between farming groups over the increasingly scarce land.
Whereas in Uruk times cities had housed a fraction of the population, the rest being dispersed over the arable lands, now quite suddenly many areas see the shift of all or most of the rural population into the towns and cities, now fortified for the first time. The development of wheeled transport around 3500 BC facilitated the peasants' daily access to their cultivated land.
It is believed that the emergence of warleaders elected to defend their cities or to lead hostile action against other city states rapidly gave rise to institutionalized kingship.
Despite its political fragmentation, however, Sumer enjoyed cultural unity, manifest in styles of pottery, temple architecture and art, in the uniformity of the emerging cuneiform ('wedge-shaped') script and in the cosmology in which each city's patron deity played his or her role, under the supreme authority of Enlil, patron of the holy city of Nippur.
Later lists of the kings of this era suggest that the rulers of many Sumerian cities recognized the (at least nominal) leadership of one of their number. The acknowledged principal city changed from time to time (as the gods directed), and included Eridu, Uruk, Kish and Shuruppak, the city of Ziusudra (the Sumerian Noah).
Leadership also passed to Ur, just after the period to which the Ur Royal Graves can be attributed. Although some of the kings named in these King Lists may have been mythical, inscriptions or seals of others demonstrate that they were genuine historical figures.
