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So what you're saying then is that the cemetary at Ur which was dated to 3500bce and all the things in it appeared out of thin air in the blink of an eye

First the Royal cemetary of Ur dates after 3000 B.C.E., and is usually dated c. 2800 B.C.E.

And no I'm not saying that it all appeared in the "blink of an eye", that is your strawman. Obviously civilization requires a period of development before hand all of which is amply evidenced by archaelogical digs in Mesopotamia.

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From the grave goods its obvious that at least two thousand years of civilisation had preceeded them.
and the sumerians king lists go back to about 5500bce for that area,

The quality or lack thereof grave goods proves little one way or the other in terms of the length of the period before they were deposited. As for the Sumerian king list, well.

a), THe authors made the assumption that each dynasty ruled the whole land, which is wrong, most of the dynasties were overlapping with other dynasties. In othert words each dynasty did not come after the pervious one.

b), The authors gave the rulers early one ridiculous reign lengths lasting in many cases thousands of years and also centuries.

The result is trying to determine relative chronology from the Sumerian king list is shall we say seriously problematic! THe first dynasty of Kish, which is generally regarded as historical is usually dated to c. 2900 B.C.E. The Kings "before the flood" are subject to serious dispute about their historical veracity.

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and the end of the Ubaidian culture that was supplanted by those who later became known as sumerians is dated to the same period. Seems like they moved in and absorbed the indigenous locals into their population.

Well it appears that the ubiad culture flourished between 5500 B.C.E., and 4000 B.C.E.

See [archaeology.about.com]

It is old hat too assume that cultural change means a population movement. As for absorbing the locals, Semitic names exist in the list of the Kings of the first Dynasty of Kish, and are common on even the eraliest cuniform documents. If the Sumerians did in fact move into the area who in fact aborbed who? It appears that throughout its history, until c. 1800 B.C.E., Sumer had a population of Semitic and "Sumerian" speakers. So the Sumerian people were in effect a "bilingual" culture / people.

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Hey presto instant class structure.

Class structure does not require civilization besides there is evidence of different classes prior to the "arrival" of the Sumerians.

As I've said before a lot depends about how you define "civilization", I was simply putting forward the "conventional" beginning of civilization in Mesopotamia.

Pierre