Longer thumbs do much more than help primates climb trees. Thumb size evolution tracks with bigger brains, which hints that hands and minds expanded together across our lineage.
A new analysis links thumb length to brain size across many primates and points to the outer brain’s thinking centers as key players. Dr. Joanna Baker of the University of Reading led the work.
Longer thumbs improve grip
Manual dexterity means precise control of the fingers to handle small objects, and it underpins tool use, food processing, and daily tasks.
Longer thumbs increase the area where the thumb meets the fingers, which improves precision grips without needing extra force.
“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 Baker.
These results connect everyday hand skills to the energetic costs of building neural tissue across evolutionary time.
Thumb length and brain size
The team focused on the first and second metacarpal bones, the long bones in the hand that sit between the wrist and the thumb or index finger.
That choice allowed scientists to compare living species with fossils, because bones preserve while behavior does not.
The new study examined 95 fossil and living primate species and tested how relative thumb length tracks with brain size.
The researchers used phylogenetic models that control for shared ancestry. The relationship stayed positive when humans were excluded, which means the signal is primate wide, not only human.
The analysis also checked whether tool-using species looked different from those that have never been recorded using tools.
There was no special offset for tool users after accounting for brain size and hand proportions, which cautions against reading tool use from one bone alone.
Thumb length links to brain’s thinking areas
The big surprise sat in which brain parts matched with longer thumbs. Instead of the cerebellum, which helps coordinate movement, the correlation showed up in the neocortex, the sheet of tissue involved in sensation, planning, and flexible control.
Earlier work documented rapid cerebellar expansion in apes, alongside changes in the neocortex. This means that the new finding does not downplay movement control, it refines where manipulation costs likely landed.
It suggests that sensing, mapping, and planning the contact between fingers and objects scales with hand skill.
In people, primary motor cortex grey matter volume is linked to how well the hand learns a new skilled movement, and disrupting this region can block learning.
That kind of result fits with the new cross-species pattern that ties fine control to cortical resources.
Tool use is not a single trait, and it varies with ecology and learning. Some primates build complex routines that require choosing, transporting, and sequencing objects.
Wild capuchins select and adjust stone hammers as nut properties change, which shows sensitive tuning rather than a one size fits all behavior.
Chimpanzees use different kits for termites versus honey, and group traditions shape how juveniles learn, but none of that maps cleanly onto one hand bone.
This is why the new result matters. It connects a simple, measurable piece of anatomy to overall brain investment while avoiding the trap of over claiming about culture from a single metric.
Unusual thumb and brain pattern
Most primates fell along the same line. Humans and our closest fossil relatives do not sit outside the broader pattern, once brain size is considered.
Australopithecus sediba’s hand stands out as it combines a long thumb with traits linked to climbing and precise grips.
Even here, the message is nuanced, because a long thumb without the full neural and skeletal package may not yield modern, human-like manipulation.
The study also checked where species defied expectations after accounting for brain size and intrinsic hand proportions.
Sediba remained unusual, while others did not, which invites deeper tests that mix simple measures with detailed biomechanics.
What this means
The link between relative thumb length and brain size supports a simple idea.
As species improved their fine control, they paid neural costs in circuits that represent the skin, the joints, and the sequences needed to handle objects.
This does not say that bigger brains came purely from tool use or that dexterity is the only driver.
It shows that everyday actions, like pinching and rotating small items, likely nudged brains to expand in networks that could keep up.
It also sets a baseline for fossils. When researchers add more species and include additional features like joint shape and muscle attachment points, they can test which combinations departed from the primate wide rule and when.
Looking ahead
Future work can map which neocortical fields scale with precision tasks across species. That could include motor and parietal areas that track finger positions in real time and combine touch with vision.
Better fossils and more high resolution scans will help resolve when different hominin lineages shifted their hands and their cortices together.
The same approach can test links between other body parts and neural systems that carry hidden processing costs.
The take home point is clear. Hands and brains changed together, and the bond runs deeper than any single tool tradition or famous species.
The study is published in Communications Biology.
—–
Like what you read? Subscribe to our newsletter for engaging articles, exclusive content, and the latest updates.
Check us out on EarthSnap, a free app brought to you by Eric Ralls and Earth.com.
—–