Images from space track relentless spread of humanity

Produced by Mairi Mackay, CNN and George Webster, for CNN, Built by Matt Barringer, CNN

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The past century has been defined by an epic migration of people from rural areas to the city. In 2008, for the first time in history, more of the Earth's population was living in cities than in the countryside. The U.N. now predicts that nearly 70% of the global population will be city dwellers by 2050.

Looking back through the decades, these snapshots from space -- created exclusively for CNN by NASA's Landsat department in collaboration with the U.S. Geological Survey -- reveal the impact of this vast population shift on cities around the world.