It seems that, for a long time, people living in cities were less able to survive and reproduce in a Darwinian sense than people who lived in rural areas. I wonder if this had a permanent impact on the gene pool. It might even be measurable someday, especially since not all parts of the world urbanized at the same time.

according to Eric Rauchway mortality due to disease was lower in cities than in rural areas for the first time around 1900 in the united states. this was due to really progressive public health measures prompted by fear of immigrant contagion. this still does not mean that cities have higher reproductive rates of course.

Besides urban fertility, the variable might also have been the rate of assimilation of migrants to the city, or an increase in the ethnic consciousness of the Semitic peoples.

Are you sure that cities had lower (surviving) birth rates back then?

In a similar fashion, the cities of Xinjiang moved from Sogdian to Turkish over a period of several centuries, starting in about 800 AD I think.

Another theory is that the soil on which the Sumerians grew their crops was becoming increasing saline. Apparently this dropped their population considerably, and the Akkadians, who were more pastoral as far as I know, were not as heavily affected.

http://jwsr.ucr.edu/archive/vol1...n3- thompson.pdf