BOZEMAN – A Montana State University paleobiologist is part of a team whose new analysis of brain evolution overturns a long-held theory about how primates, including humans, came to develop large and sophisticated brains.
The research, published this week in the journal Science, was conducted by Chris Organ, an assistant professor in the Department of Earth Sciences in MSU’s College of Letters and Science, together with a multi-institutional team of researchers. The study was led by Richard Kay, now a professor emeritus of evolutionary anthropology at Duke University, who invited the scientists, including Organ, to examine endocasts of fossil brains. Endocasts are digital, three-dimensional models of the internal surfaces of braincases that contain details about an organism’s brain anatomy, including total size, general shape, surface grooves, blood vessels and nerve exits, called foramina.
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Scientists have long known that the neocortex – the outer, folded layer of the brain responsible for sensory perception, cognition and other complex functions – is greatly enlarged in primates compared to other mammals. However, because soft brain tissue does not fossilize, exactly how and why the primate neocortex grew so large over the past 56 million years has been difficult to pin down.
Organ and the other researchers spent three years examining endocasts from 137 extinct and extant, or still living, species of primates and their close relatives dating as far back as 56 million years. Their results challenge a widespread assumption in the field.
The team found that the primate frontal lobe – the brain region most often linked to advanced cognition – grew gradually and predictably over millions of years as overall brain size increased, following the same scaling pattern across all major primate groups throughout evolutionary history. The results showed no evidence of the dramatic, independent frontal lobe expansions proposed in some earlier studies that were based on visual inspection of physical fossils.
While the data indicate that primates’ frontal lobes evolved gradually, the researchers also found evidence of dramatic expansions elsewhere in the evolving primate brain – specifically in the occipital, parietal and temporal regions, which are heavily devoted to processing what an animal sees. The findings suggest that the hallmark large brains of higher primates, including humans, are an ancient feature dating back at least 33 million years and rooted in the evolution of acute vision, rather than in selection for an enlarged, thinking frontal cortex on its own.
“Our results suggest that the enlarged brains of monkeys, apes and humans are intimately tied to vision,” Organ said. “The unique data that fossils provide here help us understand how the modern primate brain evolved.”
To gauge how much visual input ancient primates’ brains received, the team measured the size of the optic foramen, the opening through which the optic nerve passes between the brain and the eye in the skull. The two groups with the largest, most vision-dominated neocortices – anthropoids, including apes and humans, and giant-eyed tarsiers – also had the largest optic foramina, which were expected based on body size. Unexpectedly, the visual processing regions of their brains grew faster than the nerves feeding them, suggesting that magnification of small increases in visual input led to large expansions of brain tissue in the two groups.
Before examining the endocasts, the researchers developed an evolutionary tree to determine exactly how each of the 137 species was related to one another. Organ, who specializes in the study of large-scale processes and patterns in evolution, explained that the tool provided the basis for the study’s analysis.
“My research mixes data from living and extinct species to figure out how species evolve,” he said. “If we had studied just extant species in this study, we wouldn’t have made any new discoveries – fossils provide a powerful framework to understand our evolutionary past.”
In addition to Organ’s expertise, MSU contributed high-performance computational work for the study on Tempest, the largest supercomputer in Montana, which is located in the university’s Research Cyberinfrastructure core facility.
“MSU's high-performance computing cluster is an incredible research asset that can make valuable contributions to institutional partnerships like this one,” said Alison Harmon, MSU’s vice president for research and economic development. “I congratulate Dr. Organ on this groundbreaking work in paleobiology, connecting vision and the evolution of the primate brain.”
