Why do we say that it was not natural fertility and high levels of food production that were the causes of early urbanisation?
We say this because cities and towns are not merely places with large populations. It is when an economy develops in spheres other than food production that it becomes advantageous for people to cluster in towns.
Urban economies comprise trade, manufactures, and services besides food production. City people cease to be self-sufficient and depend on the products or services of other people. The division of labour is a mark of urban life.
We should not make the mistake of thinking that cities grew simply because of rural prosperity. In spite of natural fertility, agriculture was subject to hazards like floods and changing river courses. Villages were periodically relocated, and there were man-made problems as well.
Explanation
The textbook explicitly states that we should 'not make the mistake of thinking that cities grew simply because of rural prosperity.' The key argument is that urbanisation occurs when economies develop in spheres BEYOND food production - specifically trade, manufactures, and services. The division of labour is highlighted as the defining mark of urban life, not agricultural abundance. Additionally, the context notes that despite natural fertility, agriculture faced natural hazards (floods, changing river courses) and man-made problems, making it unreliable as the sole basis for urban growth.