Sumer is regarded as the first great civilization known to humanity. A. Originally, Sumerians cultivated the lower Mesopotamia region using primitive hoes and domesticated animals. B. The flooding of the Tigris and Euphrates rivers caused peasants to look to religion for explanations, resulting in the development of a large class of priests. C. Priests and warriors managed and supervised the workforce that operated the elaborate drainage and irrigation system required to maximize agricultural production. D. The emergence of new professions, sciences, and techniques and the development of trade resulting from Sumer's lack of natural resources led to a complex labor and economic system. E. The need on the part of the priestly hierarchy to keep track of who had paid taxes and tribute and how those were spent resulted in the invention of writing. F. As writing became more complex, priests increasingly put ordinary people in charge of running day-to-day operations within the Sumer economy.