> Civilizations are organized densely-populated settlements divided into hierarchical social classes with a ruling elite and subordinate urban and rural populations, which engage in intensive agriculture, mining, small-scale manufacture and trade.

Arguably a social structure is a prerequisite for a civilization. For example, you can have a village of 50 intact and trading with another village and that's not a proper civilization. There has to be the idea of a state, governance, someone entrusted to pay taxes to and perhaps some form of currency.