Science this week has a news feature by Andrew Lawler on excavations in southern Mesopotamia looking into what may be the earliest urban developments: "Uncovering Civilization's Roots" (paywall).
The riddle confronting University of Warsaw scientist Bielinski is part of an ambitious attempt to explain how humans made the momentous leap from village life to urban sprawl. That transformation first happened in Mesopotamia sometime during the 4th millennium B.C.E. in what archaeologists call the Uruk phase, named after a southern Iraq metropolis some 300 kilometers north of Bahra. But recent excavations in Kuwait, Syria, Iran, and Saudi Arabia provide mounting evidence that the origin of the urban revolution is to be found in the prior era, called the Ubaid, which began around 5500 B.C.E and lasted until about 4000 B.C.E. (see timeline, p. 792). Piecing together how and where that mysterious culture began, spread, and evolved “is a particularly hot topic right now,” says Harvard University archaeologist Jason Ur. Adds University of Chicago archaeologist Gil Stein: “This is the earliest complex society in the world. If you want to understand the roots of the urban revolution, you have to look at the Ubaid.”
There is much discussion in the article about migration versus in situ cultural intensification as triggers for urbanization. I think today it is misleading to present these mechanisms as opposed to each other. Interactions among populations were likely necessary for intensification of settlements, at the same time we know from genetics that large-scale migrations were happening in early agricultural populations.
It will be fruitful to consider how migration both introduces external change into a society but also how it spurs internal changes. I don't think that early civilizations are unique in this regard and so we should turn to a diversity of models to understand the dynamics.