If you ever find yourself in London, try to locate one of its seven gates. There you will stand where London proper ended. Everything outside that point was once a vast shantytown, a hodgepodge of terrific wooden structures and terrifying slums. More than a millennium prior to that, the Romans suffered such staggering traffic problems in their own capital that they banned delivery carts during the day. And barely two centuries ago, New York went from a sleepy backwater of hardly 30 000 souls to over half a million within 50 years: all because of a canal system.

We sometimes wonder where humans split from the so-called natural order. It began with settlements, when we started exchanging our nomadic ways for more stationary activities. The rise of specialists and elaborate social hierarchies soon followed. Over time, the settlement ‒ the city ‒ became the fulcrum of civilisation.