A new study led by a Duke University scientist overturns a long-held idea about how primates, including humans, came to have such large and sophisticated brains. The research finds that the dramatic expansion of the primate neocortex was driven largely by vision, not by the disproportionate growth of the frontal lobe often associated with higher reasoning.
The research, published this week in the journal Science , was led by Richard F. Kay , a professor emeritus of Evolutionary Anthropology at Duke University Trinity College of Arts & Sciences and the Nicholas School of the Environment .
The neocortex — the outer, folded layer of the brain responsible for sensory perception, cognition and other complex functions — is greatly enlarged in primates compared with other mammals. Exactly how and why it grew so large over the past 56 million years has been difficult to pin down, because brains do not fossilize.
To get around this problem the researchers used a unique collection of skulls, many from the Duke Lemur Center Museum of Natural History , to create models of the inside of these brain cases to compare the brains of living and extinct primates.
These models, called endocasts, were virtual. To create virtual endocasts, the scientist conducted high resolution micro-CT scans at the Duke Shared Materials Instrumentation Facility and reconstructed the 3D models digitally.
By comparing the volumes and associated surface areas of these endocasts, the team was able to see how components of the neocortex — the outer layer of the brain — evolved across primate species.
Their results challenge a widespread assumption in the field.
The frontal lobe — the brain region most often linked in popular accounts to advanced cognition — has long been thought to have ballooned independently in many primate lineages. Instead, the study found that frontal lobe size grew gradually and predictably as overall brain size increased, following the same scaling pattern across all major primate groups throughout their evolutionary history. There was no evidence of the dramatic, independent frontal lobe expansions that some earlier studies had proposed based on visual inspection of fossils.
“Everything from humans down to tree shrews, they all fall on the same line: Relative to the size of the brain, the proportion of the frontal lobe is a constant,” said Kay.
The real story, the researchers found, lies elsewhere in the brain. The occipital, parietal and temporal regions — areas heavily devoted to processing what an animal sees — expanded rapidly and disproportionately in tarsiers and in anthropoids, the group that includes monkeys, apes and humans. Crucially, those increases occurred along the same branches of the primate family tree where the optic nerve grew larger, a sign that more visual information was flowing into the brain.
“Fossil brains have been frustratingly silent on this question for a long time,” said Kay, a co-corresponding author on the study. “When we let the fossils speak quantitatively rather than relying on impressions of their shape, the enlarged brains of monkeys, apes and humans turn out to be tied to vision much more than to the frontal lobe, which simply kept pace with overall brain size.”
To gauge how much visual input ancient primates’ brains were receiving, the team used the size of the optic foramen — the bony opening that the optic nerve passes through — as a proxy. The two groups with the largest, most vision-dominated neocortices, tarsiers and anthropoids, also had the largest optic foramina, while the visual processing regions of the brain appear to have grown even faster than the nerve feeding them, magnifying small increases in visual input into large expansions of brain tissue.
“What are those visual signals for?” posits Kay. “One idea is the increased complexity of social communication. Another possibility is that their driving force is related to how efficiently these animals can forage for food. Each of those, I would imagine, could have a very strong impact on natural selection and primate evolution.”
The findings suggest that the hallmark large brains of anthropoids are an ancient feature dating back at least 33 million years and are rooted in the evolution of high-acuity vision — including adaptations such as a retinal fovea and a bony partition that shields the eye — rather than in selection for an enlarged “thinking” frontal cortex on its own.
Science
Experimental study
Not applicable
Fossil evidence favors a role for vision in the modular evolution of the primate neocortex
6-Aug-2026