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Scientists have shed fresh light on how human hands and minds evolved together, finding that longer thumbs in primates means bigger brains.
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 in the journal Communications Biology, provides the first direct evidence that manual dexterity and brain evolution are connected across the entire primate lineage, from lemurs to humans.
Thumb length and brain size connected
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.
Professor Robert Barton of our Anthropology Department is a co-author of the study.
He said: "For the first time we have been able to link two of the most distinctive features of humans, hand anatomy and brain size.
"These are key features that enable humans and other primates to literally grasp and manipulate their world."
Surprising finding
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 a brain region heavily involved in movement and coordination.
Instead, longer thumbs were connected to the neocortex.
This is a complex layered brain region comprising approximately half the volume of the human brain and which communicates extensively with the cerebellum to jointly coordinate behaviour using sensory information.
Professor Barton says it was a surprise that only one of the two major brain regions they thought would be involved in the growth process 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.
The researchers say further work is needed to establish exactly how the neocortex supports manipulative abilities.
The study was led by Dr Joanna Baker of the University of Reading, UK.
