ABSTRACT
Five thousand years ago, human beings on this globe lived in small communities of planters or animal herders. In a few areas agricultural settlements were integrated by tribal conquerors, allowing a wider division of labor and the rise of impressive civilizations. One specific challenge was to control the flow of major rivers to fertilize otherwise arid lands and to prevent flooding of settlements along the shores. Most early civilizations—such as Mesopotamia (literally, “the land between the rivers”), the Nile valley of Egypt, or the Ganges and Indus valleys of South Asia—thus developed in valleys no larger than today’s modern states.
