Scientists solve brain size mystery: humans break evolutionary trends

Rates of relative brain mass evolution. Credit: Nature Ecology & Evolution (2024).

A groundbreaking study published in Nature Ecology & Evolution has revealed that the largest animals do not have proportionally bigger brains, with humans being a significant exception.

Researchers from the University of Reading and Durham University collected data on brain and body sizes from around 1,500 species to clarify long-standing debates about brain size evolution.

For more than a century, scientists believed that brain size increased proportionally with body size.

However, this new research shows that the relationship between brain and body size is more complex.

Professor Chris Venditti from the University of Reading, the study’s lead author, explained, “We now know that the relationship between brain and body size is a curve.

This means very large animals have smaller brains than expected.”

Professor Rob Barton from Durham University, a co-author of the study, added, “Our results simplify the complex brain-body mass relationship.

We found that relative brain size can be understood using a single underlying model, which makes previously elaborate explanations unnecessary.”

This study also identified species that deviate from the norm, including humans. Our species, Homo sapiens, has evolved exceptionally large brains more than 20 times faster than all other mammal species.

But humans are not the only ones to buck this trend.

The research showed that many groups of mammals experienced rapid changes in brain size, both increases and decreases.

For example, bats initially reduced their brain size rapidly when they first evolved but then showed slow changes in brain size, possibly due to the demands of flight.

Three groups of animals showed the most significant rapid changes in brain size: primates, rodents, and carnivores. In these groups, there is a general trend of increasing relative brain size over time, known as the “Marsh-Lartet rule.” This trend is not universal across all mammals, contrary to previous beliefs.

Dr. Joanna Baker from the University of Reading, another co-author of the study, highlighted a fascinating finding. “Our results reveal a mystery.

In the largest animals, something prevents their brains from getting too big. It might be because large brains are too costly to maintain. We see a similar pattern in birds, suggesting this ‘curious ceiling’ applies to animals with very different biology.”

In summary, this research has solved a long-standing puzzle about brain size evolution. The study shows that the largest animals do not have proportionally bigger brains and that humans and some other species are exceptions to this rule.

This discovery opens new avenues for understanding how and why brain sizes have evolved differently across species.