Climatic-environmental influences on hominin brain size over the last 5 million years

A large brain relative to body mass is considered a distinguishing hominin trait. It has frequently been related to a suite of social, behavioral, technological, and other cognitive adaptations that differentiate humans from other species. The processes underlying large brain size evolution have therefore been a subject of rigorous scientific debate. Many hypotheses have been proposed to explain how climate and environment drive the selection of larger brain sizes, but monotonic influences of climate-environmental selective pressures are often assumed and rarely have between- and within-species effects been considered. Here, we apply Bayesian phylogenetic comparative techniques to the hominin fossil record to test the effect of climatic and environmental pressures (C-E) on brain size evolution, whilst simultaneously accounting for body mass and chronological age. We find that colder and more variable temperatures have a positive within-species effect on brain size evolution, likely related to biological adaptations to mitigate against hypothermia. However, in Homo, the strength of this effect diminishes over time suggesting that in later species (Homo sapiens and Homo neanderthalensis) brain sizes were less affected by C-E conditions.