Explain how Mesopotamia was home to a complex civilization that developed in the plain of the Tigris and Euphrates Rivers beginning in the fourth millennium B. C. E.

Mesopotamia is recognized as the "cradle of civilization" chiefly because of two advances that happened there, in the region of Sumer, in the 4th millennium BCE:

It is the growth of the city as we recognize that entity today. The first cities industrialized in the region known as Mesopotamia between 4500 and 3100 BCE. The city of Uruk, today considered the ancient in the world, was first settled in c. 4500 BCE and walled cities, for defense. The city of Eridu, close to Uruk, was considered the first city in the world by the Sumerians while other cities which lay entitlement to the title of 'first city' are Argos, Athens, Byblos, Jericho, Damascus, Aleppo, Jerusalem, Sidon, Luoyang, and Varasani. All of these cities are certainly ancient and are located in regions which have been populated from a very early date.

And, the invention of writing. The Sumerians first created writing as a means of long-distance communication which was required by trade. With the rise of the cities in Mesopotamia, and the need for resources which were deficient in the region, long-distance trade established and, with it, the need to be able to communicate across the areas between cities or constituencies.