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I think it has to do with population density and organisation. Hunter/gathering can't sustain a population over a certain size. Civilisation requires lots of people living and working together in a relatively small area with lots of links of organisation and communication holding everything together. You just can't do that if you're quite happy living off the land in your family unit.
Of course, organisation and civilisation improve the survival of its participating members (just as sharp teeth improve the survival rate of Tigers etc) who in turn support the maintenance of the civilisation.
It's a system of unchecked exponential growth - that is, untill it meets some kind of limiting factor. This kind of happened during the time of the Black Death, where large cities were decimated - the reason - those cities grew too large and too dirty to sustain themselves against disease. Other examples might be the World Wars of the last century, where large, opposing organised populations clashed, and the victorious form of organisation got to stamp its flavour of rules and philosophy on the world.
Beh, I'm rambling on now.
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