HUNTER-GATHERERS, who had previously made a living based on their solid knowledge of plant life and an understanding of animal behavior, continued to follow many of their old ways even as they engaged in agricultural activities. Consequently, the Neolithic Revolution, as we have come to call the invention of agriculture, although the most momentous of humankind's achievements, was not revolutionary in that it brought abrupt change. Rather, beginning about 11,000 years ago, grain gathering began to shade into grain cultivation in the Jericho Valley and, at about that time or a little later, hunting started giving way to herding in the Zagros Mountains. Millennia later surpluses were generated, giving rise to agricultural civilizations such as of Mesopotamia, Egypt, northern China, and the Indus Valley, and with them came more complex and stratified societies.

It is probably not coincidental that all of these first civilizations emerged within a relatively few centuries of one another, despite the distances separating them. Each one was located on a river and dependent on annual flooding for moist, rich soils rather than on the vagaries of rainfall. Agriculture was simplified because there was little need for plowing or manuring and, as a result, despite occasional famines, populations grew larger.