The Importance of River Valleys to Ancient Civilizations
Four of the world's ancient civilizations emerged along large rivers in dry climates, around 6000-4000 BCE, and it was not by chance. The rivers played a key role in the emergence of ancient China, the Indus Valley civilization, Mesopotamia, and ancient Egypt. Broadly speaking, the rivers not only made the civilizations possible by making food plentiful but also made them necessary by giving the people in those four areas new challenges that only "civilization" could solve.
The world's climate became drier and cooler between about 8000 and 6000 BCE. This ended a period of successful hunting and gathering by early humans in places like the Sahara, and resulted in the global pattern of deserts and fertile areas we are familiar with today. The process was slow, and the people adapted, until they found themselves largely concentrated around dwindling resources, such as large rivers in dry regions.
In those regions, because of the dwindling opportunities to gather and hunt food, the people had great need of concentrated sources of food, and this is the period of time in which people around the world, in similar circumstances, made the transition from gathering and hunting for food, to farming and husbandry. These new methods of food production allowed for more people to live in a small area than was possible without food production.
The four early river-based civilizations emerged in special cases of this, where the farming and husbandry was especially productive, and this is because of flooding. The Yellow River in northern China, the Tigris and Euphrates rivers in Iraq, the Indus River in Pakistan, and the lower Nile in Egypt are all famous for flooding. The floods were of different characters Chinese have historically referred to the Yellow (Huang) River as "China's sorrow" for its destructive power, and the ancient Greek historian Herodotus called Egypt the "gift of the Nile," because its floods were regular and fairly gentle. But all of the floods magnified the agricultural advantage by enriching the soil annually, allowing for greater concentrations of population in those four areas than around other rivers.
People in these four regions in the period of drier global climates and increased local populations responded by inventing civilization. Each group did it independently, and each invention shared characteristics with the others. "Civilization" to anthropologists and archaeologists may be simply defined as the art of living in close proximity to others. It requires rules, rulers, boundaries, special skills, economic concentration and specialization (one farmer for every few families, instead of everyone gathering and hunting), and apparently new myths and stories.
Each of the four early river-based civilizations developed the following on its own: technologies for growing and storing food and for channeling water, a system of laws to keep peace and preserve property, socioeconomic classes, theistic cosmology, writing, astronomical calendars, and other trappings of "civilization."