I had always thought that cities came about out of the basic human needs for protection (survival) & socialization, hence agglomeration. I was familiar with the fact that the first hint of urbanization appeared when Homo sapiens shifted from their nomadic lifestyle to permanent settlements. What I had never considered were the reasons that caused them to shift to permanent settlements: they understood the benefits of a network of people sharing a common outcome (that of survival & progress) through a division of labor.