History of the world
Whether in prehistoric or historic times, people always needed to be near reliable sources of potable water. Settlements developed on river banks as early as 3000 BCE in Mesopotamia, on the banks of Egypt's Nile River, in the Indus River valley, and along China's rivers. As farming developed, grain agriculture became more sophisticated and prompted a division of labour to store food between growing seasons. Labour divisions led to the rise of a leisured upper class and the development of cities, which provided the foundation for civilization. The growing complexity of human societies necessitated systems of accounting and writing.
Cradle of civilization
Around 10,200 BC the first fully developed Neolithic cultures belonging to the phases Pre-Pottery Neolithic A (PPNA) and Pre-Pottery Neolithic B (7600 to 6000 BC) appeared in the Fertile Crescent and from there spread eastwards and westwards. One of the most notable PPNA settlements is Jericho in the Levant region, thought to be the world's first town (settled around 8500 BC and fortified around 6800 BC). In Mesopotamia, the convergence of the Tigris and Euphrates rivers produced rich fertile soil and a supply of water for irrigation. The civilizations that emerged around these rivers are among the earliest known non-nomadic agrarian societies. It is because of this that the Fertile Crescent region, and Mesopotamia in particular, are often referred to as the cradle of civilization. The period known as the Ubaid period (c. 6500 to 3800 BC) is the earliest known period on the alluvial plain, although it is likely earlier periods exist obscured under the alluvium. It was during the Ubaid period that the movement towards urbanization began. Agriculture and animal husbandry were widely practiced in sedentary communities, particularly in Northern Mesopotamia, and intensive irrigated hydraulic agriculture began to be practiced in the south.