Longer thumbs mean bigger brains, scientists have found—revealing how human hands and minds evolved together.

Researchers studied 94 different primate species, including fossils and living animals, to understand how our ancestors developed their abilities. They found that species with relatively longer thumbs, which help with gripping small objects precisely, consistently had larger brains.

The research, published in Communications Biology, provides the first direct evidence that manual dexterity and brain evolution are connected across the entire primate lineage, from lemurs to humans.

Humans and our extinct relatives boast both extraordinarily long thumbs and exceptionally large brains. However, the link remains strong across all primates. When scientists removed human data from their analysis, the connection between thumb length and brain size remained.