Evolutionary signatures in deep white matter architecture: A comparative study of humans and chimpanzees

Despite near-identical genetics, humans and chimpanzees display striking cognitive differences, thought to emerge from overall brain size variation and subtle divergences in brain connectivity. We present a comparative analysis of deep white matter bundle (DWMB) morphology in 39 in vivo chimpanzees and 39 humans, using diffusion MRI and a novel isomap-based shape analysis pipeline. After mapping DWMBs into a shared anatomical space via sulcus-informed diffeomorphic registration, we identified robust species-specific differences across key frontal tracts. We focused on four frontal tracts due to their roles in fronto-parietal and fronto-temporal connectivity supporting language, executive function, and socio-emotional processing, with the arcuate fasciculus serving as an internal control given its well-established species differences. Notably, the arcuate fasciculus in humans exhibited greater curvature, volume, and temporal extension-traits absent in chimpanzees and consistent with its role in language. The uncinate and inferior fronto-occipital fasciculus revealed distinct cross-species expansions and lateralization was observed for the frontal aslants and inferior fronto-occipital fasciculus in chimpanzees. These results provide the first high-dimensional morphological mapping of DWMBs across species, uncovering evolutionary adaptations in frontal connectivity and lateralization that likely underlie human-specific cognitive abilities.