Scientists rebuilt ancient primate brains from fossils and found a stunning clue to human evolution: The brain may have grown larger not simply to think more, but because our ancestors were processing an increasing amount of visual information

For decades, scientists have faced a difficult question about primate evolution: why did the neocortex, the brain's outer layer involved in sensory perception, cognition and other complex functions, become disproportionately larger in primates than in other mammals?

The answer has been difficult to uncover because brain tissue does not fossilize.

Now, researchers have found a way to reconstruct clues about the brains of ancient primates by examining the spaces their brains once occupied inside their skulls, as per a report. Their findings, published in Science, suggest that vision played a major role in the expansion of the primate neocortex.


The study was led by Richard F. Kay, emeritus professor of Evolutionary Anthropology at Duke University. Researchers examined a unique collection of fossil skulls, most of them from the Natural History Museum of the Duke Lemur Center, as per a La Brújula Verde report.

Rather than relying only on the outside shape of the fossils, the team created detailed virtual models of the skull interiors. The results challenged the idea that the frontal lobe independently underwent rapid expansion during primate evolution.

Instead, the frontal lobe appears to have grown gradually and proportionally with overall brain size. The more dramatic changes were found in regions involved in processing visual information.

Scientists reconstructed ancient brains from fossil skulls

The researchers used high-resolution micro-computed tomography at Duke's Shared Materials Instrumentation Facility to examine the interiors of fossil skulls.

The scans produced three-dimensional digital models called endomolds or endocasts, representing the spaces once occupied by the brains.

These models allowed the researchers to compare the volumes and surface areas of different neocortex regions across living and extinct primates.

That gave the team a way to make quantitative comparisons despite the absence of preserved brain tissue. The approach also allowed the researchers to look beyond impressions based simply on the shape of fossil skulls.

The frontal lobe grew with the rest of the brain

One of the study's most important findings challenges a common assumption about primate brain evolution.

The frontal lobe is strongly associated with advanced reasoning and higher cognitive abilities. Previous research had suggested that it experienced independent and accelerated expansion in multiple primate lineages.

The new analysis found no evidence for those dramatic, autonomous expansions. Instead, the frontal lobe followed a common scaling pattern across major primate groups, growing proportionally as overall brain size increased.

Kay said that everything from humans to treeshrews fits the same general pattern: in relation to brain size, the proportion of the frontal lobe remains relatively constant, as per the La Brújula Verde report.

That suggests the frontal lobe largely kept pace with overall brain growth rather than independently driving the expansion.

The bigger change happened in areas that process vision

The researchers found a different pattern in the occipital, parietal and temporal regions of the neocortex, which primarily process visual information.

These areas expanded rapidly and disproportionately in tarsiers and anthropoids, the group that includes monkeys, apes and humans.

The expansion also occurred along the same branches of the primate evolutionary tree where the optic nerve increased in diameter.

Researchers used the size of the optic foramen, the opening in the skull through which the optic nerve passes, as an indicator of how much visual information was entering the brain.

The pattern was notable. Tarsiers and anthropoids had the largest optic foramina as well as the most vision-dominated neocortices. That connection suggests that increasing amounts of visual information were associated with the expansion of the brain regions responsible for processing it.

Visual processing appears to have amplified the effect

The researchers described what they observed as an amplification phenomenon: the visual-processing regions appeared to grow even faster than the optic nerve supplying them.

In other words, the expansion of the visual system was not simply reflected by a matching increase in the brain areas that processed those signals.

The finding points to vision as an important driver of neocortex expansion in primates. The researchers suggest that the large brains characteristic of anthropoids, dating back at least 33 million years, may have roots in the evolution of high-definition vision. That development included anatomical adaptations such as the retinal fovea and a bony partition protecting the eye.

What made better vision so important

The study does not establish one definitive explanation for what drove these evolutionary changes. Instead, Kay proposes two possible explanations.

One is increasing complexity in social communication. The other is greater efficiency in foraging and obtaining food.

Both possibilities could have placed greater demands on the visual system and increased the importance of processing visual information. The findings therefore offer a different way to look at why primate brains became so large.

Rather than assuming that the expansion was driven primarily by an independently enlarging frontal lobe, the research points toward changes in visual processing as an important part of the story, as per the La Brújula Verde report.

A new clue about how the human brain evolved

Fossil brains have long been difficult to study because the tissue itself does not survive. But the skulls containing those brains can preserve the spaces they once occupied. By turning those spaces into detailed digital models, researchers were able to compare how different regions of the neocortex changed across millions of years of primate evolution.

The results suggest that the frontal lobe did not independently drive the expansion of the primate brain. Instead, visual-processing regions grew disproportionately in tarsiers and anthropoids, including the evolutionary lineages leading to monkeys, apes and humans, alongside evidence of increasing visual input.

The findings suggest that increasing demands on the visual system may have played an important role in shaping the larger brains seen in monkeys, apes and humans, rather than the expansion being driven primarily by an independently growing frontal lobe.