Principles of neocortical organisation and behaviour in primates: Brain surfaces of 70 primate species

Dorsal view of the reconstructed cerebral hemispheres of 70 primate species. Brain surfaces include species from all major groups of primates. Colours correspond to the different families and indicate phylogenetic relationship, and brains are represented from largest on top to smallest at the bottom. The scale is the same for all brains. Preview versions are made available in portrait and landscape format, and on black and white backgrounds. Similarly, high-resolution poster versions (~A0 at 100%, 183x110cm for landscape and 119x170cm for portrait mode) are made available in both formats and both background colours, which, when printed at 45% scale, will show the brains in real size.

This figure is from our preprint:

Heuer, K., Traut, N., Aristide, L., Alavi, S. F., Herbin, M., Mars, R. B., Mylapalli, R., Najafipashaki, S., Sakai, T., Santin, M., Borrell, V., & Toro, R. (2025). Principles of neocortical organisation and behaviour in primates. bioRxiv. https://doi.org/10.1101/2025.07.17.665410

The study of brain MRI from 70 different primate species reveals a fundamental principle of neocortical organisation and behaviour driven by mechanical morphogenesis.

Abstract

The development and evolution of neocortical organisation are typically explained by the interaction of two fundamental factors: genetics and experience-dependent processes. Morphogens and signalling molecules would orchestrate the formation of neocortical areas and connections, later refined through environmental stimuli. Evolutionary changes to these genetic programmes are thought to account for the diversity of brains and behaviours observed across species. However, our phylogenetic comparative study of primate neuroanatomy and behaviour shows this view is incomplete. Using brain MRI from 70 primate species, we observed that not only the degree but also the pattern of cortical folding changes continuously with brain volume, independent of phylogenetic proximity. To better understand the consequences of this continuity, we studied New and Old World monkeys which diverged approximately 47 million years ago. Large New World monkeys, such as capuchins, have a significantly larger and more folded neocortex than many of their phylogenetically close relatives whose brains are barely folded. Notably, not only is their cortical folding pattern almost identical to that of phylogenetically distant Old World monkeys with similar brain volume, such as macaques, but also their cortical thickness maps, whole-brain connectomes, and even their behaviour. Combined analyses of MRI and endocasts from 105 primate species indicated that the highly folded neocortex of large New World monkeys evolved independently from a small, unfolded brain of their common ancestor with Old World monkeys. Remarkably, across all 70 species, behavioural similarity correlated substantially more with neuroanatomical similarity than with phylogenetic similarity. Our results challenge the prevailing explanation of the development and evolution of neocortical organisation. We propose that this challenge can be resolved by incorporating mechanical morphogenesis, alongside genetics and experience, as a third fundamental factor. Growth-driven mechanical instabilities would produce similar neuroanatomical organisation patterns and behaviours, emerging independently of the specific genetic determinants of that growth.