Well, this is a bit off the point, but I couldn't resist. It's always been a bit of a mystery why humans started living in cities -- despite what many think, city living is not, and was not, an obviously better lifestyle. However, clearly we did it, so the argument why has been partly predicated on an assumption that humans just naturally like to gather together in crowds. But it is a problem, because though there are, of course, advantages of city living, there are also disadvantages -- and the disadvantages in the initial stages tend to be more obvious than some of the advantages.

A new theory now explains it more satisfactorily, and turns conventional explanations on their head. It was thought the first great civilizations rose because the regions were particularly fertile; the new theory claims that instead it was the reduction of fertile land, water and resources in previously fertile lands, that provoked the rise of civilization. This reduction was due to climate change.