An anatomical and connectivity atlas of the tree shrew brain to bridge rodent and primate neuroanatomy.

The tree shrew (Tupaia belangeri), phylogenetically proximal to primates, serves as a critical model for evolutionary neurobiology and disease mechanisms. High-resolution MRI provides a unique opportunity to refine its neuroanatomical architecture and facilitate cross-species comparisons. Here, we present a comprehensive, ultra-high-resolution (9.4T) MRI atlas of the tree shrew brain, integrating structural and diffusion imaging to resolve fine-scale anatomical features and whole-brain connectivity gradients. Our comparative analysis characterizes the tree shrew as a distinct evolutionary mosaic: the cerebellum exhibits pronounced volumetric expansion and connectivity gradients recapitulating those of primates, whereas the hippocampus retains rodent-like architectural scaling yet preserves evolutionarily conserved longitudinal functional axes. Moving beyond these regional adaptations, we uncovered a universal organizational principle: geometry-gradient coupling (GGC)-the fundamental constraint of brain shape on functional organization. By systematically linking geometric eigenmodes to connectivity gradients across diverse species (from mice to humans), we demonstrate that despite dramatic morphological divergence, the spatial alignment between brain geometry and functional organization remains evolutionarily invariant. Collectively, these results establish the tree shrew as a pivotal phylogenetic bridge and provide a neuroanatomical benchmark for deciphering the interplay between structural diversity and universal biophysical constraints.