Hominin brain size increase has emerged from within-species encephalization

The fact that rapid brain size increase was clearly a key aspect of human evolution has prompted many studies focussing on this phenomenon, and many suggestions as to the underlying evolutionary patterns and processes. No study to date has however separated out the contributions of change through time within- vs. between- hominin species whilst simultaneously incorporating effects of body size. Using a phylogenetic approach never applied before to palaeoanthropological data, we show that brain size increase across ~ 7 million years of hominin evolution arose from increases within individual species which account for an observed overall increase in relative brain size. Variation among species in brain size after accounting for this effect is associated with body mass differences but not time. In addition, our analysis also reveals that the within-species trend escalated in more recent lineages, implying an overall pattern of accelerating brain size increase through time.