Tracing transformation across 53 million years of euarchontogliran evolution: parallelism in early primate brain shape

Essential to our understanding of primate brain evolution is the analysis of fossil brain endocasts, which allow us to trace variation through time and provide context for testing evolutionary hypotheses. The broadly held consensus that Primates are members of Euarchontoglires1 and the growing record of endocasts of living and fossil members of this group2–8 allow for the application of new approaches to answer fundamental questions relating to shifts in the shape of the endocranium with evolving function. Using 3D geometric morphometric data of living and fossil Euarchontoglires spanning 53 million years, this analysis overturns previous assumptions about primate brain evolution. We show that: 1) early fossils share a common endocranial shape, with primates diverging towards extant morphologies faster than other clades; 2) Haplorhini and Strepsirrhini share derived endocranial morphologies that evolved independently through parallel evolution; and 3) endocranial shape evolution across Euarchontoglires shows signs of constraint, possibly related to stabilising selection. These analyses demonstrate the necessity of including fossil materials when evaluating long-term evolutionary patterns and should help to form a revised evolutionary and morphological framework within which hypotheses about causality in primate brain evolution can be tested. Geometric morphometric analyses of endocasts from living and fossil Euarchontoglires reveal parallel evolution in the brain shape of early crown primates.