This Is What Our Thumbs Say About Our Brains, in a Pattern That Holds True for Other Primates

Researchers have found a link between long thumbs and big brains, suggesting the two features evolved together

Margherita Bassi | Daily Correspondent

August 28, 2025

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As the sole surviving human species, our big brains and nimble hands have always made us feel special, compared to our extinct relatives. Now, new research has associated bigger brains with longer thumbs across a wide selection of primates—the group of mammals that includes lemurs, lorises, tarsiers, monkeys, apes and humans.

In a study published Tuesday in the journal Communications Biology, a team of researchers in the United Kingdom investigated nearly 100 primate species spanning extinct and living animals and revealed that the creatures with relatively longer thumbs usually also had larger brains. The paper represents the first direct evidence that brain evolution and hand dexterity are associated throughout the primate family tree.

“Large brains and dexterous hands are considered pivotal in human evolution, together making possible technology, culture and colonization of diverse environments,” the researchers write in the study. But until now, any evidence of an evolutionary relationship between these traits was only circumstantial.

Now, “for the first time, we have been able to link two of the most distinctive features of humans: hand anatomy and brain size,” study co-author Robert Barton, an anthropologist at Durham University in England, says in a statement.

The researchers suspect that our ancestors first evolved manual dexterity, which then drove the development of a larger brain. For instance, as primates’ object manipulation improved, their brains would have had to become bigger to use these abilities, Joanna Baker, lead author of the study and an evolutionary biologist at the University of Reading in England, tells the London Times’ Rhys Blakely. “Ultimately, though, the two needed to go hand in hand, so to speak,” she adds.

Even when the team removed humans from their analysis, the trend held true across other primates. They also tested the idea that the evolution of longer thumbs is associated with tool use, but they didn’t find any correlation.

Fotios Alexandros Karakostis, a biological anthropologist at the University of Tübingen in Germany who was not involved in the study, tells the Guardian’s Nicola Davis that the research indicates that adaptations to the brain and hand likely evolved together. The study also notes, he adds, that our brain size and thumb length cannot take sole credit for our brain evolution or high level of hand coordination.

While Baker and her colleagues predicted that longer thumbs would be tied to growth of the cerebellum—the part of the brain that influences movement and coordination—the team was surprised to find that they are actually associated with the neocortex, a large region involved in cognition, consciousness and sensory information. This suggests, per another statement, that longer thumbs are “linked to thinking, not movement.” Additional research into exactly how the neocortex underpins manipulative skills could shed more light on the matter.

Ultimately, “we’ve always known that our big brains and nimble fingers set us apart, but now we can see they didn’t evolve separately,” Baker says in the statement. The study joins a host of other research involving animals that’s suggesting our species is not quite as unique as we once thought.

“We’re not saying we don’t have exceptionally long thumbs,” Baker tells the Guardian. “We do. And we’re not saying we don’t have exceptionally large brains. We do. But given the relationship between the two, that’s happening across all primates.”

Margherita Bassi | Read More

Margherita Bassi is a freelance journalist and trilingual storyteller. Her work has appeared in publications including BBC Travel, Discover magazine, Live Science, Atlas Obscura and Hidden Compass.