But if I pick berries for a living and you make hammers, suddenly we have cause to trade. Civilizations are also usually associated with social stratification, centralized government, shared values, generally in the form of religion and writing. And at least in the early days, they were almost always associated with rivers. These days you can just bisect a segment of land horizontally and vertically and boom, build a city. But 5,000 years ago, civilizations were almost always associated with rivers, whether that's the Tigris and the Euphrates, the Yellow River, the Nile, the Amazon Basin, the Coatzacoalcos. Gah, I was doing so good until I got to Coatzacoalcos! Coatzacoalcos.