By the way, I wonder how well racial IQ correlates with civilization formation. Are people of Sumerian descent unusually smart, for instance?

I have no idea how we would check this, unfortunately, short of a lot of digging up extremely old bones. The area that is now Sumeria has been swept by invasion after invasion after invasion from pretty much every direction, and over 4000 years there'd be a lot of drift even if there were no invasions and no immigration. IQ being highly polygenic makes matters worse: a few generations of dysgenic selection (extensive cousin marriage?) could wipe out much of the faint signal one is looking for, and the poor quality DNA from digs might have the same issue.