For millions of years, our distant ancestors led a precarious existence as scavengers, hunters, and gatherers, and there were fewer than 10 million human beings on Earth at any one time. But over the last two centuries, the world's population has skyrocketed. By October of 1999, some crowded cities had more than 10 million inhabitants each, and six billion people shared space on the planet. Trace the dramatic growth and spread of human populations on our global map, and see where on Earth as many as three billion more people may live by 2050.

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