Many thousands of years ago, when humanity was still in the hunter-gatherer phase, people started realizing the advantages of banding together and forming villages. Over time the rural populations of the world joined together against their common enemies: wild animals, famine, extreme weather - and the concept of urbanization was formed.

The world's population quickly started moving from a rural to an urban way of life, and by 2008 about half the world's population was living in cities. It's predicted that by 2050 around 65% of third world countries, and 86% of first world countries, will be living in cities. Already this trend has formed many mega-cities with populations over 10 million people.

These megacities have become so large that they often lead to terrible pollution, traffic, and extreme poverty. Population control is a growing issue in these cities, and, despite all this, many of these megacities have a rich culture, and special life all of their own.

Enjoyed this list? You might be interested in reading about the safest countries to live in, and the most crime-ridden cities in the world.