Study links thumb length and brain size in primates todayheadline

Our brains grew as our thumbs stretched. That is the striking conclusion of a new study in Communications Biology, which finds that primates with longer thumbs also tend to have larger brains. Using data from 95 living and fossil species, researchers at Durham University and the University of Reading report that the link holds across the primate tree, not just in humans or tool-users. The work points to dexterous hands and expanding neocortex evolving together, shaping the roots of technology, culture, and survival.

Thumbs, Brains, And Evolution

Long thumbs are key to precision grasping, the movement that lets humans hold a pen or thread a needle. Fossils show most hominins had unusually long thumbs. Yet the new analysis reveals that this trait fits within a broader primate-wide pattern: relative thumb length scales with brain size across lemurs, monkeys, apes, and even tarsiers. That means the human hand, extraordinary though it is, is part of a continuum rather than a radical departure.

“We reveal a significant relationship between relatively longer thumbs and larger brains across 95 fossil and extant primates,” the authors write in the paper.

Using Bayesian phylogenetic models, the team found the correlation robust to different statistical assumptions and not driven solely by hominins. Even when species like Australopithecus were excluded, thumb length still tracked with brain volume. The outlier was Australopithecus sediba, whose thumb was exceptionally long for its modest brain size.

The Neocortex, Not The Cerebellum

Brains are not monolithic, and the researchers probed further. In a subset of 49 species, they compared thumb length with neocortex and cerebellum volume. The surprise: thumbs correlated with neocortex size but not cerebellum size. This suggests that cortical regions tied to motor planning and sensory integration, rather than balance or coordination circuits, played the central role in shaping dexterity.

“Neural processes implicated in the evolution of manual dexterity across primates primarily involve neocortical regions,” the authors note, pointing to motor and parietal cortices.

That finding dovetails with modern neuroimaging studies showing the motor cortex expands in people who practice fine hand skills, and fossil evidence of parietal expansion in early humans. The cerebellum, despite its role in movement, appears less directly tied to thumb morphology.

Beyond Tool Use

One tempting idea is that tool use itself drove the hand–brain coevolution. But the analysis found no statistical difference between species observed to use tools—like capuchins or chimpanzees—and those that do not. Long thumbs were not a reliable predictor of tool-making. Instead, the relationship seems more general: manipulation skills of many kinds, from extracting food to grooming, carried neural costs and spurred bigger brains.

Key Findings

• Study analyzed 95 fossil and extant primates with Bayesian phylogenetic methods.
• Relative thumb length positively correlates with brain size across primates.
• Link is driven by neocortex expansion, not cerebellum size.
• Hominins fit within this general pattern, except Australopithecus sediba.
• Tool use did not predict deviations from the thumb–brain relationship.

Implications For Human Evolution

The results challenge the view that human thumbs are uniquely tied to tool culture. Instead, they highlight how neural and bodily traits coevolved over millions of years. Our extraordinary dexterity may be one endpoint of a deeper, primate-wide feedback loop: hands demanding brains, and brains enabling hands.

Future work could test more subtle morphologies—like joint shapes or tendon attachments—against brain regions mapped from fossil endocasts. For now, the message is clear. Human ingenuity rests not just in the skull but also in the bones of the hand.

Takeaway

Across primates, longer thumbs go with bigger brains, driven mainly by neocortex expansion. Humans follow this rule, rather than break it. Dexterity and cognition evolved together, long before stone tools.

Journal: Communications Biology
DOI: 10.1038/s42003-025-03320-5