Answer:
Once there was a food surplus, complexity within early river civilizations increased, as did __________, according to the archaeological record.
Step-by-step explanation:
Mesopotamia, land between rivers, its economy, as in many other fluvial civilizations, was based mainly on agriculture. The first Neolithic settlements settled in fertile areas, generated surplus agricultural and livestock that allowed them a progressive increase in population, crafts, and trade.
This transformation required the control of the course of the rivers to guarantee the crops.
The answer is: The population.