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 ...