Macroevolutionary Patterns of Endocast Lateralization in Catarrhines and Fossil Hominins

Brain lateralization has deep evolutionary roots in primates, and is often considered to reach its most pronounced expression in humans, particularly in relation to lateralized behaviors. However, little is known about the macroevolutionary dynamics of asymmetric endocranial shape across fossil and extant catarrhines. Here, we apply three-dimensional geometric morphometrics to a comparative sample of extant apes, humans, and fossil hominins to investigate patterns, rates, and directions of brain shape lateralization under an explicit phylogenetic framework.

We analyzed 161 cranial endocasts representing 81 extant and extinct Catarrhine species, using high-resolution geometric morphometrics. We focus on endocast shape lateralization as an evolutionary component of endocranial morphology rather than on individual-level hemispheric differences. Rates of asymmetric shape change are quantified across lineages by using Phylogenetic Ridge Regression (RRphylo) to map evolutionary rates of endocast shape lateralization directly onto the cortical surface.

Our results show that hominins exhibit distinctly higher rates of asymmetric endocranial shape evolution compared with nonhuman apes, with particularly pronounced changes observed along the lineage leading to modern humans. These changes are not explained by single-side brain size variation and are spatially concentrated in specific regions of the endocranial surface.

The patterns we identified reflect macroevolutionary modifications of endocranial shape and do not constitute direct evidence of hemispheric functional specialization. Nevertheless, the observed evolutionary dynamics are consistent with broader scenarios involving increasing structural reorganization of the brain during hominin evolution. These findings provide a quantitative framework for investigating the evolutionary history of endocranial asymmetry and its potential biological correlates while maintaining a clear distinction between morphological evidence and functional interpretation.