Overview

• Researchers analyzed about 94–95 living and fossil primates and found species with relatively longer thumbs consistently had larger brains.
• The association remained when human data were excluded, indicating the pattern is not driven solely by our lineage.
• Thumb length covaried with neocortex size rather than the cerebellum, pointing to links with higher-order processing over basic motor coordination.
• The authors stress that thumb length is only one proxy for dexterity and that it does not predict tool use or intelligence, with causality still unresolved.
• A separate report describes a PNAS human MRI study of more than 2,600 adults showing a similar thumb–brain correlation, which requires independent corroboration.