Hippocampal connectivity patterns echo macroscale cortical evolution in the primate brain

The hippocampus is involved in numerous cognitive functions, some of which have uniquely human aspects, such as autobiographical memory. Hippocampal anatomy, however, is typically considered conserved across primates and its evolutionary diversification is rarely studied. Comparing hippocampal structure and function is, therefore, critical for understanding human brain architecture. Here, we developed a novel comparative framework to study the hippocampus across species characterising its geometry, microstructure, and functional network embedding. In humans and macaques, we generated a new comparative space that represents the hippocampus as an unfolded surface, which respects its sheet-like anatomy. We mapped histological and MRI-derived markers of microstructure to the hippocampal surface and integrated it with low-dimensional embedding of resting-state MRI connectivity data. Our results demonstrate that the micro- and macro-structural organisation of the hippocampus are overall conserved in both species, showing consistent anterior-posterior and subfield-to-subfield differentiation. Furthermore, while hippocampal functional organisation also follows anterior-posterior trends in both species, hippocampal functional connectivity markedly reflected evolutionary reconfiguration of transmodal networks, in particular the default-mode network. Specifically, the inferior parietal lobe in the macaque mirrors an incomplete integration of the default mode network in non-human primates. By combining fine-grained anatomical investigation with large-scale functional imaging, we showed that microstructurally preserved regions like the hippocampus may still undergo functional reconfiguration, due to their embedding in higher-order association networks. SummaryWhile the hippocampus is key for uniquely human cognitive abilities, it is also a phylogenetically old cortex and paradoxically considered evolutionarily preserved. Here, we introduce a comparative framework to quantify preservation and reconfiguration of hippocampal organisation in primate evolution, by analysing the hippocampus as an unfolded cortical surface that is geometrically matched across species. Our findings revealed an overall conservation of hippocampal macro- and micro-structure, showing anterior-posterior and, perpendicularly, subfield-related organisational axes in both humans and macaques. However, while functional organisation in both species also followed an anterior-posterior axis, the latter showed a marked evolutionary reconfiguration, which mirrors a rudimentary integration of the default-mode-network in non-human primates. Our findings suggest that microstructurally preserved regions like the hippocampus may still undergo functional reconfiguration in primate evolution, due to their embedding in heteromodal association networks.