Thumb length and brain size coevolved across primates, study finds

New research comparing living and extinct primates finds a consistent relationship between thumb length and brain size, suggesting that manual dexterity and cognitive evolution developed together across the primate lineage. Scientists examined measurements from roughly 94 to 95 species, including lemurs, apes and several hominins such as Homo sapiens, Homo neanderthalensis and Homo naledi, and report that species with relatively longer thumbs tend to have larger brains. The association remained strong even when human data were excluded.

The study combined anatomical measurements with phylogenetic statistical methods to trace changes in thumb proportions and brain volume across millions of years. Researchers measured thumb length relative to other digits and compared those metrics with measured or estimated brain mass derived from brain volume or cranial cavity size. In complementary behavioural tests on a subset of species, longer thumbs predicted superior performance in moving and manipulating small objects between thumb and finger, and overall brain size also correlated with task performance.

Lead author Joanna Baker summarized the broader interpretation of the findings,...