6.1 The Origin and Influences of Urbanization

Cities need a food surplus. Once farmers could grow more than they ate (5.3), some people could live by trading, crafting, ruling or leading religious life instead of farming. Early urban hearths grew in fertile river valleys: Mesopotamia between the Tigris and Euphrates, where Uruk was a large city more than 5,000 years ago, plus the Nile, the Indus (Harappa and Mohenjo-daro), the Yellow River in China, and later Mesoamerica and the Andes.

Early cities served several functions at once: markets for trade, centers of government and religion, and places of defense. Those functions are still why cities exist.

Site is the physical character of the place where a city sits: landforms, water, soil and climate. Situation is a place's location relative to other places: trade routes, resources, neighboring cities and transportation links. A good site helps a city get started; a good situation helps it keep growing.

Urbanization is the growth in the share of people living in cities. It happens in two ways: migration from rural areas and other countries, and natural increase, when city births outnumber deaths. Several forces have pushed it forward over time:

• Agriculture: each agricultural revolution let fewer farmers feed more people, freeing rural workers to move to cities (5.4).
• Industry: the Industrial Revolution created factory jobs in cities like Manchester and Pittsburgh (7.1).
• Transportation: railroads, streetcars and later cars and highways let cities spread out and draw from larger areas.
• Communication: the telegraph, telephone and internet let companies run operations from big cities and helped world cities grow (6.3).
• Economic development and population growth: in today's lower- and middle-income countries, rural poverty and a lack of land push people toward cities that offer jobs, schools and services.
• Government policy: governments build new capitals, like Brazil's Brasília (1960), to steer growth, and some limit migration, like China's household registration (hukou) system, which ties access to many public services to a person's registered home.

How many people are urban depends on how you define a city. Countries use different definitions, so older UN figures based on them put the world at a bit over half urban. In its World Urbanization Prospects 2025, the UN switched to one method for every country, the Degree of Urbanisation. By that measure, in 2025 about 45% of the world's 8.2 billion people lived in cities, 36% in towns and 19% in rural areas. When you compare urbanization data, always check the source and the definition.