I want you to reimagine how life is organized on earth. Think of the planet like a human body that we inhabit. The skeleton is the transportation system of roads and railways, bridges and tunnels, air and seaports that enable our mobility across the continents. The vascular system that powers the body are the oil and gas pipelines and electricity grids. that distribute energy. And the nervous system of communications is the Internet cables, satellites, cellular networks and data centers that allow us to share information. Half the planet’s population lives in cities. They are the world’s engines, generating four-fifths of the global GDP. There are over 2,100 cities with populations of 250,000 people or more, including a growing number of mega-cities and sprawling, networked-city areas — conurbations, they’re called — with at least 10 million residents. As the economist Ed Glaeser puts it, “we are an urban species.”