Many early civilizations expanded into the large and complex types of societies in which most people still live today. They occurred in major river basins and developed where unique local climatic and soil conditions were favourable to produce agriculture. People of high status eventually used food surpluses as a way to pay for labour and to create alliances among groups. They began to trade and work in various skilled professions such as in political, religious, or military positions. Soon, large villages grew into urban centres that governed themselves and eventually empires covering vast territories. Secondary civilizations tended to develop in regions adjacent to these heartlands, or along the trade routes between them. The earliest civilization arose in 2500 B.C.E. in a place called Sumer of what is now present-day Iraq. It grew into a powerful and prosperous region in Mesopotamia containing 500,000 people.