For most of human history, the overwhelming majority of people lived in the countryside. As recently as 1900, fewer than one person in six worldwide lived in a town or city. By the middle of the 20th century the figure had risen to about one in three, and in 2007 the United Nations announced that, for the first time, more than half of humanity lived in urban areas. Projections suggest that by 2050 this proportion will approach seventy percent. The speed and scale of this shift have few parallels in history.
The forces driving the movement into cities are at once economic, technological and political. Modern agriculture requires far fewer labourers than traditional farming, pushing rural populations to look elsewhere for income. Factories, offices and services concentrated in cities create jobs that are, on average, better paid than those available in villages. Young adults are also drawn by the prospect of higher education, more varied entertainment and, in some countries, greater personal freedom.
The consequences are difficult to evaluate in simple terms. Cities are engines of economic growth, innovation and cultural exchange. They make the efficient provision of services - water, electricity, transportation, medicine - possible on a scale that rural areas cannot match. At the same time, rapid urbanisation has produced vast informal settlements, where residents live without secure housing, clean water or adequate sanitation. Urban inequality can be extreme, with high-rise luxury buildings a few streets from districts of acute poverty.
Planners and policy makers therefore face a distinctive challenge. The growth of cities is unlikely to slow, yet the conditions in which that growth occurs are largely a matter of choice. Thoughtful investment in transport, housing and public services can make cities healthier, more sustainable and more equitable places. Neglect or corruption can produce the opposite. In that sense, the question of the 21st century is less whether humanity will become an urban species - it already has - than what kind of cities it will build.