Determination and explanation of the regularities underlying these independent courses towards similar outcomes is the province of Anthropology. Some explanations work well for some civilizations but not for others. Canal systems require centralized control of construction, maintenance and water allocation. Irrigation agriculture can sustain the large populations. This model works well for Mesopotamia and Egypt.
Survival requires a central authority and military hierarchy.
Centralized control of production and distribution of food, salt, metals, essential raw materials and finished products is needed to maintain an orderly flow of resources and finished goods from areas where they are produced to those areas where they are needed. The only raw materials they had were plants, mud and water. The demand for simple commodities such as stone and salt led to systematic exchange of agricultural produce for such materials, and the emergence of an administrative structure to direct exchanges and redistribute goods. Those with better access to resources, for example irrigation water, can increase production greatly and accumulate surpluses. This may be a foundation for manipulating others through supplying food.