 The Uruk period is a continuation and an outgrowth of Ubaid with pottery being the main visible change.By the time of the Uruk period (c. 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.