What makes Mesopotamia an advanced and complex society?

Mesopotamia means "land between rivers". The rivers are the Tigris and Euphrates. There were thousands of cities there and we have no idea whether they were all one society or something else. We call the society Sumer and the people Sumerians just to have something to call them. Whatever you call them, you are reducing several thousand years of history to a single word.

The definition of a complex society is not very specific but generally means some sort of division of labor. "Advanced" has no strictly defined meaning.

Now here is this:
io9.com/more-than-half-of-the-worlds-population-lives-inside-t-493103044