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After spending tens of thousands of years living mostly in small settlements, humans have entered an urban stage of evolution. As of 2008, more than half the world's people live in cities, and the urban population is swelling by 1 million every week. By 2030, almost 6 in 10 people will live in metropolitan areas, which exert a powerful pull as economic and social magnets.
That concentration of people gives rise to some of the world's greatest problems, such as air and water pollution, poverty-stricken slums and epidemics of violence and illness. Yet throughout history, urbanites have produced...