Maybe it's my inner Brooklynite talking, but to me cities are natural phenomena: Ants make hills, bees make hives, humans make cities. Each of these colonies is a meta-organism, really. They all must survive by taking in resources and then figuring out what to do with the waste that comes out the other end. The most innovative organisms and communities figure out ways to recycle waste into raw material for more housing, new appendages, or further growth.