Primates with longer thumbs tend to have bigger brains, research finds

"We imagine an evolutionary scenario in which a primate or human has become more intelligent, and with that comes the ability to think about action planning, think about what you are doing with your hands, and realise that actually you are more efficient at doing it one way or another, said Dr Joanna Baker, lead author of the research from the University of Reading."

"Large brains and manual dexterity are both thought to have played an important role in human evolution, with opposable thumbs a key feature that enabled a greater ability to grip and manipulate items including tools. I discovered chimps using tools and people wouldn't believe me However, with some other primates having partly opposable thumbs, questions have remained over whether other changes in the hand such as thumb length could also be important in the evolution of tool use."

Analysis of estimated brain mass and thumb length across 94 primate species, ranging from ancient hominins to lemurs, indicates a positive association between relatively longer thumbs and increased brain size. Humans and most other hominins display thumbs markedly longer than expected from general primate hand proportions. Longer thumbs increase the range of motion for picking up and controlling small objects, improving precision grip and manipulation necessary for tool use. Enhanced manual dexterity would favor selection for action planning and cognitive control of hand movements. The overall pattern supports a scenario in which enlargement of the brain and improvements in thumb morphology co-evolved in primates.

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