The book applies Dunbar's hypothesis of the social brain and his theory of time budgets to human evolution over the last 5 million years, up to modern humans. The effective brain volume or neocortex ratio, which we know in primates is strongly related to group size, is the main variable upon which Dunbar founds a big body of theory on social behavior in the hominid lineage, including the evolution of culture, religion, and language. This all looks very speculative. What conclusions on social behavior you can draw from fossils and artifacts is quite limited.