Longer thumbs mean bigger brains and this is “pivotal” to human evolution, research has found.
Scientists studied 94 fossils and living animals to understand how our ancestors developed their gripping abilities.
The study, published in the journal Communications Biology, looked at primate species from lemurs to neanderthals and found the brain co-evolved with manual dexterity to help with gripping small objects.
“Large brains and dexterous hands are considered pivotal in human evolution, together making possible technology, culture and colonisation of diverse environments,” the research authors said.
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.”
In tests on 41 primate species, scientists found that those with longer thumbs could move small objects more easily between their thumb and finger, and also found the size of their brain predicted how well they performed these tasks.
Scientists expected longer thumbs to be linked with the cerebellum - the part of the brain responsible for movement and coordination.
But instead, they found longer thumbs were connected to the neocortex - a complex, layered region which takes up half the volume of the human brain. This area processes sensory information and handles cognition and consciousness.
These 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.
However, study authors explain that thumb lengths alone cannot be used to identify tool use, with the relationship of brain and thumb size constant across all primates, regardless of whether they used tools.
“This is in line with suggestions that features of the hominin hand, including long thumbs, pre-date the origin of systematic tool production,” the study authors wrote.
“Our results provide no support for the idea that thumb lengths are sufficient morphological indicators of tool-use –either in hominins or across all primates. We therefore cannot make any inferences about tool-use in hominins from our results.”
The authors say further work is needed to establish exactly how the neocortex supports manipulative abilities.
