Most primate societies are independent, single-group structures. In contrast, human sociality involves multilevel, nested alliances.
Close-knit mixed-sex nonhuman primate groups last only until sexual maturity. To avoid interbreeding either males or females emigrate to another group.
In contrast, humans remain familial into adulthood. Pair-bonding between families created affines (in-laws) that extend group cohesion. This created kin-based interlocking bonds between groups, forming the basis for tribalism that still comprises the core building block of human social structure. Upon this larger societies were built. Further social glue was applied with shared culture and aligned economic interests.