"As for Mespotamia your date is a probably too early the usual dates given are 3500 - 3000 B.C.E. "
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
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,
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.
Hey presto instant class structure. at that time the geography of the country dictated that they had to have come from the north, unless you're considering ocean migration prior to 5500bce which is a little unlikely. the reed boat technology that followed was only made possible by the local reed marshes in the first place...so unless they were swimming......