Chapter 3 
Migration and Empire: the Spread of Civilization

As the early river valley civilizations grew, they also continued to develop new and improved agricultural tools. Perhaps the most significant such tool was the plow, which made it possible to bring more and more land under cultivation. The silts deposited in river valleys thanks to periodic flooding were already moist and therefore easily worked by human hands. Beyond the reach of such floods, however, most land quickly became dry and therefore much harder to work. Even in areas of regular rainfall, drier harder soil did not easily absorb enough of the rainwater to allow large-scale surplus agriculture to develop. As often as not, most rain simply ran off the land without penetrating it. Such dry conditions also made it difficult to actually plant seeds in the ground, and if seeds were simply scattered on the surface most would be eaten by birds and foraging animals before they could germinate and take root. By using plows to break up such hard dry soils, however, people were gradually able to expand large scale agriculture beyond the river valleys into all areas that received enough rainfall to make crops possible. Expansion of agriculture and growing populations into rain-watered regions also led to increased trade. Consequently, between about 3000 B.C. and 1400 B.C. the expansion of large-scale agriculture outside the river valleys also made it possible for civilization itself to expand into any area that could sustain sufficiently productive rainwater agriculture.