Our two civilizations of the Western Hemisphere are mostly alike and very different from the others. Their formative periods began in the middle to late second millennium B.C, as a result of maturing agricultural techniques. In both cases, advancing agriculture was fused with religious belief systems to produce chiefdoms.
We see in all of these civilizations the catalysts of human development: fertile soil producing from the most primitive farming techniques and eventually a surplus of crop leading the way to a differentiation of human skill and the population density of a city. Urbanism was, in the beginning, protective against the invader, but later evolved according to local forces.
Data from the chart came from Elman's book Origins of the State and Civiliza