Longer Thumbs Linked to Bigger Brains in Primates

Primates with longer thumbs also tend to have larger brains, according to new research that sheds light on the co-evolution of hand function and cognitive capacity.

Published in Communications Biology, the study examined 94 primate species, both living and extinct, to assess how thumb length relates to brain size. The results suggest that improved grip and manipulation skills evolved in parallel with cognitive complexity throughout primate evolution.

A cross-species link

Humans and their extinct relatives possess unusually long thumbs and disproportionately large brains. However, the researchers found that the association between thumb length and brain volume remained statistically robust even after removing human data from the analysis. This suggests the evolutionary link is not unique to humans.

The research team focused on a key anatomical trait: the length of the thumb relative to other fingers. This feature is considered critical for precision grip, an ability important for tool use and object manipulation. The study revealed that across species, those with relatively longer thumbs consistently had larger overall brain sizes.

Brain region involved in cognition, not just movement

The researchers also investigated which brain regions were associated with thumb length. They initially hypothesised that the cerebellum, which coordinates movement and balance, would be linked to manual skills. Instead, they found that variation in thumb length corresponded more closely with the size of the neocortex.

The neocortex plays a role in higher-order brain functions, including perception, conscious thought and decision-making. The finding suggests that as manual abilities advanced, primates developed enhanced cognitive processing to support them. However, the exact nature of this relationship remains unclear and is the focus of ongoing research.

Reference: Baker J, Barton RA, Venditti C. Human dexterity and brains evolved hand in hand. Comm Biol. 2025;8(1):1257. doi: 10.1038/s42003-025-08686-5

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