It’s hard to pin down the precise moment the world’s center of gravity shifted. For thousands of years, people lived in the countryside. They worked on farms or in villages, knew little of the world beyond their immediate families and neigh­bors, and generally got by on their own. Slowly, they began to congregate. It happened in Mesopotamia and Egypt, later in Greece and Rome, and also in Europe and the Americas. More recently, we’ve seen fast growth in Africa and, most spectacularly, in Asia. And then, by 2008, according to the United ­Nations, the balance finally tipped: in the ebb and flow of daily births and deaths, the number of people who inhabit the world’s cities ticked into the majority, for the first time ever.