Abstract
Large brains and dexterous hands are considered pivotal in human evolution, together making possible technology, culture and colonisation of diverse environments. Despite suggestions that hands and brains coevolved, evidence remains circumstantial. Here, we reveal a significant relationship between relatively longer thumbs - a key feature of precision grasping - and larger brains across 95 fossil and extant primates using Bayesian phylogenetic methods. Most hominins, including Homo sapiens, have uniquely long thumbs, yet they and other tool-using primates conform to the broader primate relationship with brain size. Within the brain, we surprisingly find no link with cerebellum size, but a strong relationship with neocortex size, perhaps reflecting the role of motor and parietal cortices in sensorimotor skills associated with fine manipulation. Our results emphasise the role of manipulative abilities in brain evolution and reveal how neural and bodily adaptations are interconnected in primate evolution.