Podcast with Jon Kaas on brain evolution and neocortex

Why does a duck-billed platypus have electroreception, and what does that tell us about how 250 million years of evolution sculpted the six-layered cortex that makes you human? Jon Kaas traces the entire arc of mammalian brain evolution from stem reptiles to primates.

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Kaas argues that understanding brain evolution is essential for understanding what we are. All mammals share a six-layered neocortex that evolved from a simpler one-layered dorsal cortex in stem reptiles, a structure likely involved in habituation and short-term memory rather than sensory processing. The transition to six layers gave early mammals extraordinary flexibility: the ability to replicate cortical areas, specialize them for different functions, and modify sensory representations by enlarging what matters most, whether whisker maps, nose representations, or echolocation frequencies.

The interview reconstructs the ecological pressures that drove early mammalian brain evolution. Small, nocturnal, and hunted by dinosaurs, the first mammals developed high-frequency hearing through the dissociation of jaw bones into inner ear ossicles, enabling mother-offspring communication at frequencies predators could not detect. Olfaction dominated the forebrain, essential for nocturnal foraging. Sensory hairs, likely the precursors of whiskers, provided tactile information before physical contact, a critical advantage in poor light. Kaas emphasizes that the brain's hyperplasticity, its ability to automatically reorganize when peripheral inputs change, was the key innovation enabling rapid diversification.

The primate chapter of this story centers on the shift to diurnal, arboreal life after the dinosaur extinction 60 million years ago. Visual processing expanded massively, with the temporal and occipital lobes growing to cover the midbrain. Eye-hand coordination became critical for catching insects on moving branches, driving the development of grasping forepaws and eventually freeing the hand from the mouth. Social group living, essential for ground-dwelling primates facing predators, drove frontal lobe expansion. Throughout, Kaas stresses that cortical and subcortical structures co-evolved, with changes in cortex driving modifications in spinal cord circuitry, thalamic inputs, and midbrain organization.