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 people started farming, these 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 lives and ways of thinking.
Travelers and explorers came back from the corners of the earth with fantastic stories and new ideas and technologies. Their adventures, and they were true adventures, offer insight into the newly connected world.