A large literature characterizes urbanisation as resulting from productivity growth attracting rural workers to cities. Incorporating economic geography elements into a growth model, we suggest that causation runs the other way: when rural workers move to cities, the resulting urbanisation produces technological change and productivity growth. Urban density leads to knowledge exchange and innovation, thus creating a positive feedback loop between city size and productivity that initiates sustained economic growth.
This jives with the Steven Johnson book that says how much more innovatition increases in big cities. People together is a powerful thing.