Mystery of brain size and human uniqueness solved

MADRID (Euronews) – Larger animals do not have correspondingly larger brains, and humans are bucking this trend, according to a new study published in the journal Nature Ecology & Evolution.

Researchers from the Universities of Reading and Durham have compiled a massive dataset of brain and body size from around 1,500 species to clarify centuries of controversy surrounding the evolution of brain size.

Large brains relative to body size are associated with intelligence, sociability and behavioral complexity, and humans have developed unusually large brains. New research shows that larger animals do not have correspondingly larger brains, challenging long-held views on brain evolution.

“For more than a century, scientists have assumed that this relationship is linear, meaning that the bigger the brain, the larger the brain,” Professor Chris Venditti, lead author of the study at the University of Reading, said in a statement. “We now know that this is not true, and that there is a curve between brain size and body size, which basically means that very large animals have smaller brains than expected.”

Professor Rob Barton, co-author of the study from Durham University, said: “Our findings help to untangle the puzzlingly complex relationship between brain and body mass. The simplicity of our model means that the explanations of relative brain size outlined above can be investigated using a single underlying model.”

Beyond the Ordinary

The study revealed a simple correlation between brain size and body size across all mammals, allowing researchers to identify norm breakers: species that go against the norm.

These outliers include our own species, Homo sapiens, which evolved more than 20 times faster than all other mammalian species, resulting in the huge brains that characterize humans today. But humans are not the only species to buck this trend.

All mammal groups show rapid changes, either toward smaller or larger brain sizes. For example, bats experienced very rapid brain size reductions when they first emerged, but then changed their relative brain size very slowly, suggesting possible evolutionary constraints related to the demands of flight.

Three groups of animals have shown the most dramatic changes in brain size: primates, rodents, and carnivores. In all three groups, there is a trend for relative brain size to increase over time (the “Mashr-Rath rule”). This is not a universal trend across all mammals, as previously thought.

Co-author Dr Joanna Baker, also from the University of Reading, said: “Our results reveal a mystery. In larger animals, there is something that stops the brain from growing too large. It was unclear whether this is because brain size rises above a certain range. But as we also observe similar curvature in birds, the pattern appears to be a universal phenomenon: what causes this ‘strange upper limit’ to apply to biologically very different animals?