Summary: The evolution of complex societies began when agricultural subsistence systems raised human population densities to levels that would support large scale cooperation, and division of labor. All agricultural origins sequences postdate 11,500 years ago probably because late Pleistocene climates we extremely variable, dry, and the atmosphere was low in carbon dioxide. Under such conditions, agriculture was likely impossible. However, the tribal scale societies of the Pleistocene did acquire, by geneculture coevolution, tribal social instincts that simultaneously enable and constrain the (...) evolution of complex societies. Once agriculture became possible, a competitive ratchet drove further improvements in subsistence and in scale of social organization . Those societies that grew and became better organized were advantaged in individual wealth and economic and military power, and tended to conquer, absorb, or be imitated by smaller and less well organized societies. Internal competitors for power espousing useful social innovations could deliver improved returns when their quest was successful. Notwithstanding the ratchet, social complexity increased only slowly in the first half of the Holocene and even afterwards few periods except the past two centuries saw changes that were dramatic on the scale of individual lifetimes. We attempt a taxonomy of the processes that regulate rates of institutional evolution, cause reversals of complexity against the ratchet, and impose historical contingency on institutional evolution. (shrink)