Longer thumbs, bigger brains — hands and minds evolved together.
A new study of 94 living and fossil primate species finds a clear link between relatively longer thumbs and larger brains, suggesting manual dexterity and brain expansion evolved in step across the primate family tree. The result holds even when humans and their close relatives are removed from the analysis, meaning the pattern isn’t just a human special case.
“We’ve always known that our big brains and nimble fingers set us apart, but now we can see they didn’t evolve separately. As our ancestors got better at picking up and manipulating objects, their brains had to grow to handle these new skills. These abilities have been fine-tuned through millions of years of brain evolution,” says Dr Joanna Baker, lead author, University of Reading.
The researchers measured thumb and finger bone proportions (the first metacarpal and related elements) across a broad sample of primates and paired those measurements with published brain-size data. Using phylogenetic comparative models that account for evolutionary relationships, they tested associations between thumb length and whole-brain size, and — where data allowed — between thumb length and major brain regions.
Their analyses show a consistent positive relationship between relative thumb length and whole-brain size. When the brain was partitioned, thumb length correlated with the neocortex — the layered outer region involved in sensing, planning and higher cognition — but not with the cerebellum, which is commonly associated with coordination and motor control. That distinction was unexpected and points toward cognitive or integrative processing changes accompanying improved manual skills.
The authors are careful not to claim a simple cause–effect rule: longer thumbs are likely one of several anatomical and neural changes that enabled advanced manipulation. They note the fossil record and regional brain data are still incomplete, so further work tracing timing and functional links will strengthen the picture.
Citation:
J. Baker et al. Human dexterity and brains evolved hand in hand, Communications Biology. Published online August 25, 2025. doi:10.1038/s42003-025-08686-5
