It is not until long after the beginning of the historical period where we have significant definitive linear culture which is the continuing evolution of population growth and increased trade brought about by the Ubaid. In a relatively short period of time Ubaid material culture spread over an enormous geographical area which urban centers became more numerous and populated. The key point is that just because Ubaid material culture is found does not mean they are "Ubaid", just that these other peoples were influenced by this contact at the very least on a material level creating a "homogeny" of sorts. As great as the Uruk expansion was, for example, it ended despite the fact their material culture lived on and evolved in other cultures.
The idea that civilization advanced in a linear fashion is somewhat of a misnomer as before such long lasting continuous cultures like Dynastic Egypt, Babylon, and China came about, I think most, including concurrently with these cultures, would be best characterized as successive. For example, Akkad was already their own thing, a tribal confederate, before they took over Sumer and it was only after that they became unified, the "Akkadian Empire", which after the fact having absorbed Sumerian culture continued on their merry way as something "new". Same thing with Assyria and Babylon after Akkad fell. There were already people there which they absorbed Akkadian culture, augmented by migrants and expats, and became something more than they were before. I think what is linear is the medium of civilization not necessarily the people who later came to benefit from it.
Edited 3 time(s). Last edit at 07-Oct-17 03:56 by Thanos5150.