What is the primary cause of the significant increase in population and social complexity during the agrarian stage of sociocultural evolution?

The primary cause of the significant increase in population and social complexity during the agrarian stage of sociocultural evolution is the development of agriculture. When people learned to farm, they could produce more food than they needed for just their families. This extra food allowed them to support larger communities and led to the growth of villages and towns. For example, in ancient Mesopotamia, the ability to grow crops like wheat and barley meant that people could settle in one place rather than moving around to find food. As communities grew, they developed more complex social structures, including leaders, trades, and laws, which further increased their social complexity.