Homo erectus likely disappeared from Java about 150,000 years ago, and Homo sapiens did not resettle the island until the last ice age about 10,000 years ago, when the polar ice caps receded to their present extent. The record of human habitation in China is considerably longer. There, fossils of Homo sapiens thought to be 150,000 to 200,000 years old have been found. By 20,000 years ago, modern humans probably lived throughout China.
The earliest known civilizations arose in the great river valleys of southwest Asia, northwest India, and northern China. Despite differences, these cultures had some similar characteristics. All were agricultural societies that depended on advanced social and political structures to maintain irrigation and flood control systems. Raids by nomadic herders forced farmers to live in walled cities for defense and to entrust their protection to aristocratic leaders. The invention of the plow about 3000 BC increased farm productivity and reduced the need for farm labor, freeing workers to become artisans. An increased agricultural yield and the work of the artisans provided trade goods that could be exchanged with people from other cultures. (Adapted from Encarta Encyclopedia)