Early humans had pretty small social networks. At most, they probably met only a couple hundred people who probably all lived very similar lives to their own. As agriculture took hold in each civilization, these social networks got larger. People were increasingly specialized in their work and trade.
Populations in cities got larger. Trade reached across longer distances, bringing together people with very different ways of thinking. All of these elements combined to speed up the process of collective learning. It's not that humans necessarily got smarter. There simply were more of them and they got better at sharing information.
Enhanced forms of communication in the form of writing were developed, and eventually humans were able to print large quantities of what had been written. The means of transportation became more sophisticated and included domesticated animals, ships, and systems of roads that made it easier to cover long distances.
Agrarian (farming) civilizations did not exist entirely on their own. As they grew and stretched their boundaries, they joined up with other civilizations. Together, they formed larger civilizations. The linking up of different civilizations was an important process. It guaranteed that collective learning reached further. As collective learning grew, it embraced more people and greater diversity than ever before.
Significant exchanges of trade and ideas occurred in all the world zones. But the most important exchange networks emerged in Afro-Eurasia. This world zone included Africa, Europe and Asia. At this time, the zones still were isolated from each other. Humans living in one zone knew nothing of events in the others.
In Afro-Eurasia, all agrarian civilizations linked up into a vast interconnected network called the Silk Roads. The roads connected people by land around the beginning of the Common Era. This network wasn’t just about the trade of goods. Travelers on the road also spread social, religious, and philosophical ideas. They learned each other’s languages, and exchanged technology.
Because of specialization in labor, more jobs developed to accommodate the needs of the people and the civilization. As societies interacted with other civilizations through trade, the expansion of ideas and exchange of goods fostered even more job opportunities.
The byproduct of these new jobs was the creation of multiple levels of social classes. People were grouped into various levels based on their value and importance in society, their power, or their wealth. Often times, the social structure in ancient civilizations looked like a pyramid, due to the smallest number of people holding the greatest power (the top) and the largest number of people having the least amount of power (the bottom).