Why did civilisation begin between two rivers?

Civilisation began there because the dry south could only be farmed with river water brought by canals, and that work needed many people organised together. Mesopotamia, "the land between rivers" in Greek, sits between the Tigris and the Euphrates, but its climate is semi-arid. In the north there is enough rain for farming. In the south, where the first cities arose, crops only grow with irrigation: water carried from the rivers along canals.

That one fact shaped everything. Digging and cleaning canals takes many hands working to a plan, and historians note that the need for this labour helped urban settlements and centralised authority develop from the earliest period. The people of Sumer, and later Akkad, built their cities along the rivers and their branches. The rivers gave fish for food and fertilizer, reeds, and clay for building, and with irrigation the food supply was comparable to that of the Canadian prairies.

All of this sits inside the Fertile Crescent, believed to be the first region where settled farming emerged. Farming there let early civilisations such as Sumer flourish, and writing, the wheel and glass mostly appeared first in Mesopotamia.

By the late fourth millennium BC the south was a patchwork of independent city-states, divided by canals and boundary stones. Each was centred on a temple to its patron god or goddess. Their quarrels, which were recorded on tablets, were often about land and canals.

The same rivers could also turn on their people. They sustained life, but frequent floods ravaged entire cities. And over the centuries poorly drained irrigated fields in the far south grew saltier, which slowly pulled power north.