Asia
The coastal periphery was home to some of the world's earliest known civilizations, each of them developing around fertile river valleys. The civilizations in Mesopotamia, the Indus Valley and the Yellow River shared many similarities. These civilizations may well have exchanged technologies and ideas such as mathematics and the wheel. Other innovations, such as writing, seem to have been developed individually in each area. Cities, states and empires developed in these lowlands.
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.