Why do people crowd into cities at all?

People crowd into cities because that is where the work, the schools and the other people are. A city is a permanent, densely packed place whose residents mostly do something other than farm. That last part is the key: a city can only exist when the countryside around it grows more food than its own farmers need.

In the usual account, that surplus is why cities came so late. For thousands of years, nearly everyone worked the land and only small market towns existed. Most archaeologists call Uruk, in what is now Iraq, the first true city, around the mid-fourth millennium BC. Later Sumerians told stories about their hero Gilgamesh building its walls. Inside a city, people could specialise and trade instead of farming. Farmers sent grain in, and the city sent back money, administration, manufactured goods and culture.

Then the balance broke. In the late 18th century, farming and industry changed so fast that the share of people in cities exploded. In England and Wales it went from 17% in 1801 to 54% in 1891. Worldwide, only 15% lived in cities around 1900, and in 2007 the world passed 50% for the first time.

People keep coming for the same reasons: more jobs to choose from, better schools, shorter trips between the things they need. Cities have a cost too, including stress and high living costs. But squeezed into about 3% of the planet's land, dense cities, with efficient transport and little land per person, can have a smaller ecological footprint per person than sprawling places.