During my undergraduate studies at the University of Nevada, Reno I had a Peninsular Medieval Literature class focused on early Spanish literature of the Iberian Peninsula. Today, the Iberian Peninsula contains two sovereign countries, and Spain contains four dominant sub-cultures. But in the past, the Peninsula had many different tribal cultures separated geographically and separated in terms of how they interacted with outsiders. Over time, through trade, conquest, and other means, the tribes coalesced to form the starting blocks that became Portugal, Spain, and the minor sub-cultures that exist within the countries.
This pattern of cultural agglomeration has been common throughout human history. At least since the Agrarian Revolution, living in larger tribes has been advantageous for humans. The evolution and growth of our brains and social institutions has created an environment that favors larger numbers. Consequentially, human societies and cultures have been on a pathway toward coalescence. As Yuval Noah Harari writes in his book Sapiens, “Over the millennia, small, simple cultures gradually coalesce into bigger and more complex civilizations, so that the world contains fewer and fewer mega-cultures, each of which is bigger and more complex.” Harari acknowledges that this is a generalization, and that even mega-cultures maintain sub-cultures and smaller segments that may break apart, but the trend seems to hold with dominant institutions taking root across the smaller sub-cultures.
Humans originally evolved within small tribes. The brains of our earliest ancestors did not have the capability to maintain large social groups, and cultural evolution had not provided humanity with institutions that could maintain large groups. The earliest humans, likely similar to ape groups of today, could only maintain social cohesion among so many members before the group broke apart. The evolution of Homo Sapiens set humanity on a new path where the human brain could support ever larger and more complex social organizations, ultimately favoring larger cultures and more complex cultural agglomeration for a host of reasons that are beyond the scope of this post.