What drove the dramatic expansion of the primate brain? A study led by Richard Kay, professor emeritus of evolutionary anthropology at Duke, published in the journal Science, challenges the long-held assumption that the frontal lobe, the seat of planning and higher reasoning, powered that growth. The fossil record, the team argues, points somewhere else entirely: to vision, and to the brain regions that process what the eyes take in.
Brains do not fossilise, so the researchers turned to skulls of living and extinct primates, many held in the Duke Lemur Center Museum of Natural History. Using high-resolution micro-CT scans, they built digital models of the interior of each braincase, allowing brain structures to be compared across species and tracked across millions of years of evolutionary time, including fossils such as Simonsius grangeri.
The frontal lobe did grow as overall brain size increased, but in no primate group did it grow faster than other regions. The most striking changes came instead in the occipital, parietal and temporal regions, areas that process visual information, which expanded rapidly in tarsiers and in anthropoids, the lineage including monkeys, apes and humans. Those changes coincided with larger optic nerves, evidence that ever more visual input was reaching the brain.