How the Human Cerebellum May Outshine Our Prefrontal Cortex
Humans' extraordinary brainpower may be tied to the evolution of our cerebellum.
The evolutionary growth of humans' prefrontal cortex has long been considered a singular explanation for our extraordinary brainpower.
However, accumulating evidence suggests that previously unknown evolutionary changes to the human cerebellum make our brains unique.
Recent evolutionary studies disrupt the status quo and raise new questions about how humans' cerebral and cerebellar brain regions evolved.
Historically, neuroscientists and evolutionary anthropologists assumed that modern humans' disproportionately large frontal lobes and the explosive growth of our prefrontal cortex millennia ago were the deciding factors that gave us significantly more brainpower than archaic humans and chimpanzees.
Because we have more prominent frontal lobes than Neanderthals and our non-human primate cousins, most evolutionary studies have focused on modern humans' prefrontal cortex as the primary reason our brains are so extraordinary.
Have We Been Overestimating the Evolutionary Significance of Our Prefrontal Cortex?
However, a few years ago, groundbreaking research ( Neubauer et al., 2018 ) into the evolution of modern human brain shape found that noteworthy bulging of both hemispheres in the cerebellum (as marked by enlarged, rounded cerebellar areas at the base of the skull) gave present-day modern humans "globular brains and globular endocasts."
Around this same time, another pioneering study ( Kochiyama et al., 2018 ) used computational neuroanatomy technology to reconstruct 3D brain shapes of Neanderthals and early Homo sapiens . This study also showed that early Homo sapiens had a much bigger cerebellum than Neanderthals.
As Takanori Kochiyama and co-authors explain, "Such a neuroanatomical difference in the cerebellum may have caused important differences in cognitive and social abilities between the two species and might have contributed to the replacement of Neanderthals by early Homo sapiens ." (See " Bigger Cerebellum Size May Have Helped Early Humans Thrive .")
In recent weeks, two different studies drive home the possibility that the evolution of our cerebellum may have played a more important role in the evolution of humans' exceptional thinking capacity than previously believed.
The Human Cerebellum Takes Center Stage as Key Evolutionary Player
In the first study ( Weiss et al., 2021 ), researchers from Stanford University and UCSF devised a novel method for harvesting information from the genomes of archaic humans, such as Neanderthals and Denisovans, and comparing the genomic differences between modern-day humans and them. Their findings were published on April 22 in the peer-reviewed journal eLife .
"This work provides insight into the regulatory function of variants that emerged along the modern human lineage and the recent evolution of human gene expression," Carly Weiss and co-authors write. Notably, Weiss et al. found that genes affecting the cerebellum "might have been particularly affected by these expression changes, which is in line with previous comparisons based on the fossil record [described above] and DNA methylation. More importantly, these results provide candidate sequence changes underlying these evolutionary trends."
"We also identified an enrichment of brain-related phenotypes, particularly those affecting the size of the cerebellum," Weiss et al. note. "Studies of introgressed sequences have also shown that the cerebellum is one of the regions with the most divergent expression between Neanderthal and modern human alleles. Together with our results, these data collectively suggest that sequences separating archaic and modern humans are particularly linked to functions of the brain, and especially the cerebellum."
Compared to Monkeys and Chimps, the Human Cerebellum Has Distinctive Epigenetic Features
The second recent brain evolution study ( Guevara et al., 2021 ) compared epigenetic modifications to DNA in the cerebellum of humans, chimpanzees, and monkeys. This comparative analysis by researchers at Duke University reveals that the human cerebellum has distinctive epigenetic features compared to non-human primates. These peer-reviewed findings were published on May 6 in PLOS Genetic s.
"The cerebellum—a part of the brain once recognized mainly for its role in coordinating movement—underwent evolutionary changes that may have contributed to human culture, language, and tool use," the PLOS news release explains. "Scientists studying how humans evolved their remarkable capacity to think and learn have frequently focused on the prefrontal cortex, a part of the brain vital for executive functions, like moral reasoning and decision making. But recently, the cerebellum has begun receiving more attention for its role in human cognition."
Guevara and her Duke colleagues compared differences between the cerebellum and prefrontal cortex on a molecular level. Their research showed that methylation patterns in human DNA were significantly greater in the cerebellum than in the prefrontal cortex. This suggests that there may have been more evolutionary changes to cerebellar brain tissue than cerebral brain tissue.
Notably, a side-by-side comparison of epigenetic features in chimpanzees, rhesus macaques, and human brains showed that the human cerebellum had greater epigenetic differences than our prefrontal cortex.
"The epigenetic differences were especially apparent on genes involved in brain development, brain inflammation, fat metabolism, and synaptic plasticity—the strengthening or weakening of connections between neurons depending on how often they are used," the news release states.
"Our results support an important role for the cerebellum in human brain evolution and suggest that previously identified epigenetic features distinguishing the human neocortex are not unique to the neocortex," Elaine Guevara concludes.
Although the prefrontal cortex has long been the primary focus of research related to genes associated with optimizing human intelligence and offsetting cognitive decline, the latest evolutionary and epigenetic research suggests that future studies should continue to delve deeper into how the structure and functional connectivity of the human cerebellum affects our thinking.
Christopher Bergland is a retired ultra-endurance athlete turned science writer, public health advocate, and promoter of cerebellum ("little brain") optimization.