Dunbar’s number
Humans, just like any other animal, manage a simple objective function with two factors: energy and gene perpetuation, or more simply food and sex. These two factors compete for resources that can be proxied generally by the limited allocation of time afforded to each individual. Maximizing utility then simplifies into assuring sufficient food while successfully perpetuating genes, before she runs out of time.
An individual operating in a large theater had substantial risks. Fifty thousand years ago in Africa, the possible theater was all humanoids in the continent. Accumulation of food required hunting in collaboration with others and the expedition benefited from both the size of the group and the ability to coordinate. The size of the group may have had ambiguous net effects because of benefits from more shots on goal from a larger group negated by added complexity in coordination. However, the need for sharing the spoils with all involved unambiguously pointed to reducing the size of the hunting party. On the need of gene perpetuation, an individual is likely more successful if she narrowed the scope and pursued with more focus. Thus, it is clear that early humans would have always preferred to narrow the scope and size of their theater of operations.
Human clans seldom exceeded 150 members. In the modern context, this could be understood by Dunbar’s number that is suggested to be the cognitive limit to the number of people with whom one can maintain stable social relationships — relationships in which an individual knows who each person is and how each person relates to every other person [1]. This number also appears to play a major role in companies, showing a higher chance of failure when employees swell close to it in small companies and fragmentation and a rise in internal politics among different groups of that size in large companies [2].
Segmentation is a necessary condition for humans. The pyramidal structures they erect from countries, religions, languages, race, color, height, weight, gender, political parties, sports teams, and neighborhoods are built brick by brick, each with less than 150 members.