Agricultural development led early societies to settle. Settlements required growing food and a lasting source of water. That's why these early civilizations were born around rivers. The proximity of rivers facilitated access to fertile soil and, logically, to water.
Once a civilization has a stable source of food it can start growing surplus of it, which creates the possibility of trade. That is, stable food production is a base for the complexification of a society that can lead to the creation of other jobs, like the merchant.
It also enables a society to complexify its social structures, to build cities, to develop religion and culture which creates more jobs.
