Due to the accumulation of wealth, an assured food supply and an increasing division of labour, people were able to live together in large numbers and the first permanent cities appear. This process was fostered and assisted by the discovery and development of metallurgy (at first copper, then its alloy with tin, bronze), as well as the art of writing. It was in this phase of development also that the wheel was invested.
There are two schools of thought on all human inventions. One, the 'infusionist' who claimed that nothing was ever invented twice. That all human inventions stemmed from a single source and that through a process of diffusion they spread to all part of the world.
