Huge modern cities didn't exist before fossil fuels and only a handful of cities of any size existed before modern agriculture.
It is interesting to me that in recent history, say the last 500 years, there were only a few large cities but various 'oligists go on and on about the huge cities of ancient times, 1-2-3 million people in Rome for example. Whereas in modern times it was not until the 18th century that London reached 1 million. Strange.
I think we tend to forget that the "great" cities of the distant past were very special cases, reliant upon an excess of power for their survival just as modern cities. The it was human slave power and or military power able to force tribute from subsistence farmers (in the form of food) to feed non-farming urbanites. Basically they were command economies. They simply could not have existed otherwise because there wasn't the technology that would allow a surplus of food - a voluntary surplus that is.
I actually think that lots of the great ancient cities were mere symbols of power, political or religious, some surely were densely inhabited for a time but many only sparsely.
Anyway aside from however the ancients did it, up to the 18th century the vast majority of the population lived on the land as subsistence farmers with only a vanishing minority in cities, again because there was not the technology to grow a surplus that would allow specialization and trade and manufacturing that constitutes and is the excuse for a city. They may have slept in a village of a few dozen families or inside the walls of a fortification but they were subsistence farmers nonetheless.
Jethro Tull invented the seed drill in 1700 and that really was the beginning of modern cities. In the 18th & 19th century there was finally enough food surplus to allow populations to grow without the need for slave labor. Obviously slave labor was still profitable but eventually the civilizing effect of a full belly brought most overt slavery to an end.
So in addition to being the seat of power, whether government, religious or military, after the first ag revolution they became increasingly the center of growing trade and manufacture.
But again I think we forget that it is only in the last couple of hundred years that cities really become common centers of population. Even as late as 1900, only 13% of the world's population lived in cities and not until 2007 that the percentage exceeded 50%. In the US the 50% mark was passed in the 1910's
We also tend to forget (perhaps because we live in the city and forgetting is convenient) that fossil fuels are the slaves enabling the huge population of modern cities. Fossil fuel extraction over the last 300 years has enabled the growth and very existence of cities that would not and can not survive otherwise. http://www.gizmag.com/go/7334/ http://www.localhistories.org/farming.html http://en.wikipedia.org/wiki/City