Primate thumbs and brains evolved hand-in-hand

Longer thumbs mean bigger brains, scientists have found - revealing how human hands and minds evolved together.

Researchers studied 94 different primate species, including fossils and living animals, to understand how our ancestors developed their abilities. They found that species with relatively longer thumbs, which help with gripping small objects precisely, consistently had larger brains.

The research, published today (Tuesday, 26 August) in Communications Biology, provides the first direct evidence that manual dexterity and brain evolution are connected across the entire primate lineage, from lemurs to humans. 

Humans and our extinct relatives boast both extraordinarily long thumbs and exceptionally large brains. However, the link remains strong across all primates: when scientists removed human data from their analysis, the connection between thumb length and brain size remained.

Dr Joanna Baker, lead author from the University of Reading, said: “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.”

Thumbs linked to thinking, not movement

The scientists made a surprising discovery about which part of the brain grows alongside longer thumbs. They expected longer thumbs to be linked to the cerebellum because it is the region of the brain that controls movement and coordination. Instead, longer thumbs were connected to the neocortex (a complex layered region comprising approximately half the volume of the human brain), which processes sensory information and handles cognition and consciousness.

It was a surprise that only one of the two major brain regions they thought would be involved actually was. The findings suggest that as primates developed better manual skills for handling objects, their brains had to grow to process and use these new abilities effectively - but further work is needed to establish exactly how the neocortex supports manipulative abilities.

Baker, J., Barton, R. A., & Venditti, C. (2025). Human dexterity and brains evolved hand in hand. Communications Biology, 8(1), 1-9. https://doi.org/10.1038/s42003-025-08686-5