"Up until 10,000 years ago there were no permanent settlements and all human groups lived by hunting and gathering. Then agriculture was discovered and everything changed. Now a small number of people could supply food for the rest and the first cities arose. Every since that time there has been a steady movement of people out of our original arcadia and into cities, such that now over half the world lives in them. But why given that cities have historically been targets of attack and places of crime and where diseases fester and spread? The answer is that cities have acted as gardens of our prosperity, creativity and innovations and their continued existence is vital to fitting the projected 9 billion people onto this planet. Surprisingly, they are the new 'green centres' of the world."