The Sumerian civilization, one of the earliest recorded, began with the early agricultural revolution when humankind learned to produce a surplus of crops and animals. This not only led to the development of sedentary societies, but also to specialization of labor and tasks. In time, specialization and non-nomadic societies developed unique characteristics such as language, culture, etc. In the case of the ancient Sumerians, this process began sometime around 6,000 B.C.E. They developed in the Fertile Crescent of Mesopotamia. Their society became so specialized that cities in the area were able to extend their political and military control outside the city limits.
A key to their development appears to be the development of irrigation systems for crops. This allowed them to grow necessary foodstuffs without dependence on precipitation.