The agricultural revolution and the first cities

For most of human existence people lived by foraging: hunting, fishing and gathering wild plants. Groups were small, mobile and relatively egalitarian, because possessions have to be carried and a surplus cannot be stored by people who move.

Foragers were not poor in knowledge. Their diets were varied, their working hours were often shorter than those of early farmers, and their skeletal remains show fewer signs of nutritional stress. This matters because it rules out the simple story that farming was adopted because it was obviously better.

Agriculture appeared independently in at least seven regions, including the Fertile Crescent, the Yellow River valley, Mesoamerica, the Andes and New Guinea. The timing clusters after the last ice age, when a warmer and more stable climate made reliable harvests possible.

The advantage was not quality of life but quantity of people. Farming produces far more calories per unit of land, so farming populations grew and expanded into the territory of foragers. Once a group depended on crops it could not easily return, since its numbers now exceeded what wild food could support.

Stored grain is the hinge of early history. A surplus means some people need not produce food, which allows full-time specialists: potters, smiths, soldiers, priests and administrators. It also means there is something worth defending, worth taxing and worth stealing.

Writing appears in this context and its earliest uses are administrative, not literary. The first Mesopotamian tablets record quantities of barley, sheep and labour. Law codes such as Hammurabi's follow, formalising rules that had previously depended on memory and personal authority.

• Narrower diets built on a few staple crops, producing deficiency diseases visible in skeletons.
• Crowding plus domesticated animals created the conditions for epidemic disease.
• Stored wealth allowed durable inequality, since some families could accumulate what others could not.
• Dependence on a small number of crops made famine possible when a harvest failed.
• Labour demands rose, and evidence of repetitive strain injuries increases sharply.

Uruk in southern Mesopotamia reached tens of thousands of people by around 3000 BCE. Comparable centres emerged at Mohenjo-daro in the Indus valley, along the Nile, and later at Teotihuacan in Mesoamerica.

Cities share a recognisable set of features: monumental construction, craft specialisation, long-distance trade, record keeping and a social hierarchy expressed in housing and burial. They also share vulnerabilities: dependence on surrounding farmland, on water management, and on the legitimacy of whoever collects the taxes.