The extraordinary evolution of humans is often exemplified by two defining traits: large brains and highly dexterous hands. Together, they enabled the development of tools, the rise of complex cultures, and the ability to adapt to a wide range of environments.
Some researchers have proposed that these traits evolved in tandem, each influencing the development of the other. The idea is compelling: as our ancestors began to manipulate objects with increasing precision, the cognitive demands of tool use and social coordination may have driven the expansion of their brains. Conversely, a growing brain could have enabled more sophisticated hand use.
However, despite the appeal of this coevolution hypothesis, the evidence remains largely circumstantial. Fossil records and archaeological findings suggest a correlation between the two, but they do not yet provide definitive proof that the evolution of the brain and hands was directly linked in a cause-and-effect relationship.
A new study published in Communications Biology sheds light on how human dexterity and intelligence may have evolved together. Researchers analyzed 94 primate species, both living and extinct, using a Bayesian phylogenetic comparative approach. This method combines evolutionary history with statistical modeling to test relationships between physical traits and behavior.
They discovered a consistent link: species with relatively longer thumbs, which aid in precise gripping, also tended to have larger brains. It suggests that manual dexterity and brain evolution are connected across the entire primate lineage, from lemurs to humans.
Notably, the correlation held even when human data were excluded, indicating that this pattern is not unique to our species.
"We've always known that our big brains and nimble fingers set us apart, but now we can see they didn't evolve separately," explained Joanna Baker, lead author on the study. "As our ancestors became more adept at picking up and manipulating objects, their brains had to adapt to handle these new skills. These abilities have been fine-tuned through millions of years of brain evolution."
Large portions of the primate neocortex and cerebellum are dedicated to visuo-motor control, coordinating what we see with how we move. The coordinated expansion of these brain regions accounts for much of the variation in brain size across primate species. Given their role in fine motor skills and spatial processing, these areas were expected to play a key role in the coevolution of manual dexterity and brain size.
Researchers investigated the relationship between these two brain regions (the neocortex and the cerebellum) and thumb length. Using models that tested the effects of both regions simultaneously, they found a significant positive relationship between thumb length and both finger length and the neocortex.
In contrast, the cerebellum showed no such association. Notably, these findings remained consistent even when each brain region was analyzed independently, reinforcing the neocortex's unique association with manual dexterity.
This pattern likely reflects the specialized roles of the motor and parietal cortices in sensorimotor integration, particularly in tasks requiring fine manipulation.
"Our results emphasize the role of manipulative abilities in brain evolution and reveal how neural and bodily adaptations are interconnected in primate evolution," the researchers conclude in the study.
The new study was published in Communications Biology.
Source: University of Reading
