It would seem that history has the answer, or at least one that makes sense to me.
Not long ago I watched a documentary called God, Guns and Germs.
The root message was that Civilizations that travel laterally are more successful then those that travel vertically due to two reasons.
1. Similar Eco systems support current cultural practices i.e. hunting, foraging etc.
2. Encounters with rivals produce defense mechanisims which force broad thinking and innovation.
Civilizations, or cultures that become xenophobic (be it from walls, or location) always fall behind. China is a perfect example, as are the Americas. When we consider the Cities of Mesopotamia, and other Cultural cross roads, we most also consider population, predation (fellow humans, or predators) density, trade, and religion. Walls are built to keep the bad guys, and the critters out. For the lack of large predators, or humans. Only a ditch with some pointy stakes or some other less formidable obstacle is required.
The larger the population, the more inclined a government of some kind will evolve toward urbanization. Often times it is a result of Religion. In my humble opinion, some wise guy/girl convinces his/her neighbor to start doing the chores, and poof! a priest/king is born. But the basis for the cooperation in the first place is more due to commerce, common protection and culture then for anything else.
We are social creatures after all. We protect our own, but once others start moving in and begin to over hunt/fish/farm, we start thinking about our own in smaller terms. Then things start getting rough. I think a vast majority of our problems have been over looked because it is so ingrained in us by religion, and social ideas that we have overlooked that which is most apparent. There is just to many of us, and there has been for a Very long time. Which raises one very good question. What now?
I know I went a bit a field here, but...well. There it is

"If your system of beliefs were responsible for all the worst atrocities in human history, would you really want to know about it?"