(obviously I will need to do more research before publishing this article, maybe reread the section on money in Sapiens, and get a bunch of other sources as well)
The exchange of valued items is one of the backbones of human civilization. In the hunter gatherer era, tribes would trade with one another, ideally not only for each other’s material benefit, but to also build psychological and emotional bonds with one another. One tribe may be good at making an excess amount of animal skin clothing, and another may be good at making an excess amount of sharp hunting tools and these would be traded.
This method of trading worked in small scale societies of no more than a few hundred members. Deals could be made by the leaders of the tribes to determine what was a fair trade. But during the agrarian age, as civilization began to increase the number of inhabitants of a settlement to thousands or even tens of thousands, the problems with directly exchanging physical goods began to show. How many knives are worth one fur coat? How many nuts are worth sending someone to get a bucket of water from the river a 30 minute walk away? How many mangos are worth a house? These do not have straightforward answers. Different people will have different opinions on the values of these items, and to avoid violent conflict disputing these a new method seemed likely necessary.
Enter money, a fictitious vehicle designed to trade valuable items between people. Money by itself has no value, and it only works because the people in a civilization who use it believe it to have value. Let me be clear, money does not solve all of the issues with direct trading, it also introduces a host of new problems, but after millennia of testing it appears that in a large-scale civilization of at least a few thousand people, having an imaginary value system solves more problems than it creates.