The coevolution of encephalization and manual dexterity in hominins and other primates

The human hand is one of our most remarkable features. We have long, opposable thumbs and a suite of other features argued to be adaptations for interacting with and manipulating our environment, literally extending the reach of our cognitive powers. Consequently, enhanced manipulative dexterity, tool use and increased brain size are considered key features of how our ancestors evolved. This hypothesis predicts that anatomical dexterity and brain size co-evolved, and that this should be evident in the morphology of the hand. To test this hypothesis and to understand how hominin evolution may have deviated from any general primate trend, we collected a dataset of finger length, thumb length, and brain size across 94 extinct and extant species of primates. Using phylogenetic comparative methods, we reveal a strong primate-wide association between brain size and relative thumb length. We further demonstrate that this general association accurately predicts the co-evolution of these traits in hominins. Whilst hominins have significantly long thumbs compared to other primates, we infer that they have arisen from the same underlying evolutionary process acting across the whole primate order: increasing manipulative ability associated with specialized neural control processes. The relationships we recover are consistent with positive feedback between manipulation, tool use and cognition and therefore may go some way towards explaining evolutionary trends towards larger brain sizes among primates, and particularly in hominins. Our results emphasize the role of manipulative abilities in cognitive evolution and emphasize how neural and bodily adaptations are interconnected.