bronze usage most probably started in caucasia, since the caucasions warriors ruled the mesopotamian cities temporarly with the swords in their hands. but it does not mean they had higher level of civilisation. the civilisation started in mesopotomia, egypt, india and china since they were all located near the big overflowing rivers. there were no knowledge of agriculture depending on rain in that time.
specialists are musts for the cities. you need specialists to built buildings, priests to satisfy goddess (in agriculture civilization it is mostly goddess referring to the fertility, instead of gods), rulers, clerks etc. who will not work in the farm. so, there must be excess food for them provided by the farmers. it was possible only in the lands mentioned above to create that much excessive food for the rest of the cities.
nomads life was more simple but wilder, they were following climates without having cities with less amount of social rules.
their wild life made them conqueror against peaceful agriculture cities.
this cycle (agriculture and livestock or goddess and god) war had been worked more then 5000 years (see huns, mongolians).
sorry for my english.