For many years, scientists have adhered to a straightforward principle: larger bodies mean larger brains. A fascinating new study has challenged this idea, indicating that brain size can only increase to a certain extent, though this exception does not seem to have applied to humans. Our brains have evolved 23 times faster than those of other mammals, resulting in much larger brain sizes. Although it was previously believed that brain size scaled proportionally with body size, the study suggests there may be an upper limit to how large brains can grow.

Researchers from the University of Reading and Durham University have gathered an extensive dataset of brain and body sizes from approximately 1,500 species to address long-standing debates about brain size evolution. They’ve published their research in the latest edition of Nature Ecology and Evolution.




Evolution of the human brain from bigger to bigger still and then suddenly 3,000 years ago it starts to shrink. (kotjarko / Adobe Stock)

Evolution of the human brain from bigger to bigger still and then suddenly 3,000 years ago it starts to shrink. (kotjarko / Adobe Stock)

Brain Expansion: An Evolutionary Accident?

In the past two decades, it’s been discovered that small-brained hominin species coexisted with their larger-brained counterparts for a considerable time. Growing evidence indicates that these small-brained species exhibited complex behaviors. For instance, some crafted intricate stone tools, likely requiring language skills.

It appears that brain expansion began as an evolutionary accident, which then triggered further changes, causing this growth to escalate. Interestingly, the factors that drove this expansion might also account for a recent 10 percent reduction in human brain size. This raises the possibility that our brains could continue to shrink, potentially leading to humanity’s downfall!

Professor Chris Venditti, lead author of the study from the University of Reading, said in a press release:

“For more than a century, scientists have assumed that this relationship was linear – meaning that brain size gets proportionally bigger, the larger an animal is. We now know this is not true. The relationship between brain and body size is a curve, essentially meaning very large animals have smaller brains than expected.”

Humans are not the only species that defy the traditional correlation between body and brain size. Various mammal groups exhibited rapid changes in brain size, both increases and decreases. For instance, bats significantly reduced their brain size quickly after evolving, then experienced very slow changes in relative brain size. This indicates that the demands of flight might have imposed an evolutionary constraint on their brain development.

Responding to Fight or Flight: Nature’s Creatures

Three groups of animals—primates, rodents, and carnivores—demonstrated the most pronounced rapid changes in brain size. In these groups, relative brain size has tended to increase over time. However, this is not a universal trend across all mammals, as once thought.

Posterior distributions (transparent lines) and medians (solid lines) of the model predictions demonstrate the trend in three mammalian orders (rodents, yellow; carnivores, red; primates, pink). Silhouettes indicate the relevant taxonomic groups and are not to scale. (Venditti, C. et al/Nature)

Posterior distributions (transparent lines) and medians (solid lines) of the model predictions demonstrate the trend in three mammalian orders (rodents, yellow; carnivores, red; primates, pink). Silhouettes indicate the relevant taxonomic groups and are not to scale. (Venditti, C. et al/Nature)

The study also revealed a limiting factor preventing brains from becoming excessively large in bigger animals, possibly because maintaining very large brains requires too much energy. Interestingly, similar patterns were observed in birds, suggesting this phenomenon might be common across different species, according to a press release by Durham University.

“Our results reveal a mystery. In the largest animals, there is something preventing brains from getting too big. Whether this is because big brains beyond a certain size are simply too costly to maintain remains to be seen. But as we also observe similar curvature in birds, the pattern seems to be a general phenomenon – what causes this ‘curious ceiling’ applies to animals with very different biology,” provided Dr Joanna Baker, co-author of the study from the University of Reading.

A History of Big Braining: From Asia to Africa

Twenty years ago, researchers in Indonesia discovered Homo floresiensis, a 1.1-meter (3.6 feet)-tall human with a tiny brain of 425 cubic centimeters. Astonishingly, this species survived until about 50,000 years ago, indicating that small-brained humans coexisted with our species long after it emerged more than 300,000 years ago.

Homo floresiensis wasn’t alone; about five years ago, Homo luzonensis was found in the Philippines, surviving until around the same time. Additionally, Homo naledi, discovered in 2013, had a brain volume of no more than 550 cubic centimeters and lived in southern Africa until at least 235,000 years ago, alongside larger-brained humans!

These discoveries reveal that small-brained hominins were capable of sophisticated behavior. For instance, Homo erectus at the Gona site in Ethiopia, around 1.6 million years ago, produced Acheulean stone tools, including complex hand axes. These tools were so difficult to make that their creation likely required rudimentary language skills. Surprisingly, a 2020 study showed that some of these toolmakers had brains less than half the size of modern humans, reports The New Scientist.

Further evidence comes from the Nyayanga site in Kenya. Researchers found Oldowan tools, up to 3 million years old, associated with Paranthropus fossils, a hominin with a brain volume as low as 450 cubic centimeters. Despite its small brain, Paranthropus may have made these tools.

Why Big Brains?

Big brains, while advantageous, also pose significant challenges. They consume about 20 percent of our daily energy intake, despite comprising only 2 percent of our body mass. Additionally, birthing and raising large-brained babies is difficult.

One hypothesis suggests that hominins living in larger social groups needed bigger brains to navigate complex social environments. However, this theory is problematic, as researchers have not found a consistent correlation between brain size and social group size across primate species. Another hypothesis has suggested eating meat on a more regular basis created the energy over time to develop a bigger brain, though this single contributing factor too seems implausible.

In fact, it is scientifically simplistic to attribute brain expansion to a single factor. Improved diets and larger social groups likely contributed, but a more comprehensive explanation involves a positive feedback loop between brain size and behavior. Small-brained hominins might have developed new tools and basic language skills, enhancing their survival chances. As slightly larger-brained individuals mastered these technologies and behaviors more effectively, average brain sizes would have increased. This, in turn, facilitated further improvements in tool-making and language skills, driving further selection for larger brains.

In conclusion, researchers now entertain the idea that societal and technological changes can influence evolution and shape our brains. For some, the big question is whether this is always to our benefit. This has become a particularly pertinent point with respect to Artificial Intelligence (AI) and our relationship with creating an abiotic species that may replace humans. Again, perhaps like other technologies developed by the human brains, AI and human beings are likely to coexist in a larger history of evolutionary developments.

Top image: Human brain Anatomical Model illustration.            Source: paul/Adobe Stock

By Sahir Pandey

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