  Empirical evidence suggests that social structure may have changed from
hierarchical to egalitarian and back along the evolutionary line of humans. We
model a society subject to competing cognitive and social navigation
constraints. The theory predicts that the degree of hierarchy decreases with
encephalization and increases with group size. Hence hominin groups may have
been driven from a phase with hierarchical order to a phase with egalitarian
structures by the encephalization during the last two million years, and back
to hierarchical due to fast demographical changes during the Neolithic. The
dynamics in the perceived social network shows evidence in the egalitarian
phase of the observed phenomenon of Reverse Dominance. The theory also predicts
for modern hunter-gatherers in mild climates a trend towards an intermediate
hierarchy degree and a phase transition for harder ecological conditions. In
harsher climates societies would tend to bemore egalitarian if organized in
small groups but more hierarchical if in large groups. The theoretical model
permits organizing the available data in the cross-cultural record
(Ethnographic Atlas, N=248 cultures) where the symmetry breaking transition can
be clearly seen.
