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
The Norte Chico civilization proper is understood to have emerged around 3200 BC, as it is at that point that large-scale human settlement and communal construction across multiple sites becomes clearly apparent. Since the early 21st century, it has been established as the oldest known civilization in the Americas. The civilization flourished at the confluence of three rivers, the Fortaleza, the Pativilca, and the Supe. These river valleys each have large clusters of sites. Further south, there are several associated sites along the Huaura River. Notable settlements include the cities of Caral, the largest and most complex Preceramic site, and Aspero. Norte Chico sites are known for their density of large sites with immense architecture. Haas argues that the density of sites in such a small area is globally unique for a nascent civilization. During the third millennium BC, Norte Chico may have been the most densely populated area of the world (excepting, possibly, northern China). The Supe, Pativilca, Fortaleza, and Huaura River valleys each have several related sites.