The first farmers depended on rain: where crops would grow could shift within a generation as annual rainfall varied. In the great river valleys of the Near East — Tigris, Euphrates, Nile — even where rain was scarce, an annual flood deposited fresh, moist soil every year. From around 6000 BCE in what is today southern Iraq, Iran, and Egypt, humans took the next step: the flood's water was channelled with small dykes and canals onto specific fields. Grain no longer depended on rain but on engineering.

The result is much more than a simple yield bump. Irrigated land yields three to five times more grain per area than rain-fed land, and is far more resilient against drought. The surplus reached a scale that could support people who did not work the fields at all: artisans, priests, soldiers, scribes. On top of these specialists came urbanism, hierarchy, and eventually the state. The great urban civilisations that follow in Mesopotamia — Sumer, Akkad — are the direct children of irrigation engineering.

Maintaining a canal requires shared labour: who will dig, who will repair, who is first in line for water this season? Irrigation is not only a technique but a form of governance. That demand for governance is one reason the first written law codes — Ur-Nammu, Hammurabi — devote so many clauses to water rights and property. Water created the first bureaucracy.