What if the story of human intelligence begins with our thumbs?

That’s the idea behind a new study that has scientists rethinking the evolutionary partnership between hands and brains. After sifting through data from 95 living and extinct primates, a team from the University of Reading and Durham University found a striking pattern: species with relatively longer thumbs consistently had bigger brains.

“We’ve always known that our big brains and nimble fingers set us apart, but now we can see they didn’t evolve separately,” said Dr Joanna Baker, the study’s lead author. “As our ancestors got better at picking up and manipulating objects, their brains had to grow to handle these new skills”.

Hands That Think

The human thumb rides on a saddle-shaped joint at the base of the hand. That shape lets it swing, swivel, and roll across the palm to meet each finger — a move called opposition. Those motions come from the trapeziometacarpal joint plus a cluster of thenar muscles that power fine control. Put together, they turn a stubby digit into a multitool.

But our thumbs aren’t just clever hinges. They’re part of a bigger story about brains. The researchers expected that longer thumbs would connect to the cerebellum, the brain’s coordination center. Instead, they found the strongest link was with the neocortex — the sprawling brain region that processes sensory information, plans actions, and fuels consciousness.

Dexterity didn’t just give our ancestors better grip; it demanded more advanced thinking. Dr Baker explained: “We imagine an evolutionary scenario in which a primate or human has become more intelligent, and with that comes the ability to think about action planning, think about what you are doing with your hands.”

Even when the scientists removed humans from the dataset, the pattern held across lemurs, monkeys, and apes. Long thumbs predicted large brains.

Still, there were exceptions. Australopithecus sediba, a two-million-year-old relative, had unusually long thumbs even after accounting for brain size. Its anatomy suggests a mix of tree-climbing and tool-handling features, perhaps an evolutionary experiment that didn’t last.

Beyond Tools

The obvious question is whether this is all about tool use? After all, opposable thumbs have long been celebrated as the gateway to hammers, spears, and smartphones. But the data tell a subtler story. Tool-using primates didn’t show a special bump in the thumb–brain relationship. Capuchins, for example, crack nuts with stones despite having only partly opposable thumbs.

The implication is that manual dexterity itself — not just tool-making — helped drive brain growth. Extracting food, handling objects, and manipulating environments all demanded neural resources.

Still, thumb length is only one piece of the puzzle. Other features of the hand — like finger proportions, joint mobility, and muscle attachments — also shaped dexterity. “Thumb length and brain size alone could not fully explain or represent human-like manual dexterity,” Dr. Fotios Alexandros Karakostis of the University of Tübingen, who was not involved in the study, told The Guardian.

The findings remind us that our minds are not the products of brains in a vat, but the result of bodies in motion. As proponents of “embodied cognition” argue, the brain evolved in constant conversation with the body’s capabilities. This study puts that idea into literal perspective. The size of our thumbs may have nudged the size of our brains.

It’s humbling to think that every text you send, every coffee cup you hold, every tool you wield, rests on this ancient feedback loop. We don’t just think with our brains — in many ways, we also think with our hands.

The new findings appeared in the journal Communications Biology.