How thumbs played a key role in shaping human brains and intelligence

New research shows that primates with longer thumbs also tend to have larger brains. The finding suggests that brains and hands evolved together, not on separate tracks.

The study covered 94 primate species, living and extinct. From lemurs to humans, the same trend appeared: longer thumbs, bigger brains. That pattern stayed strong even when humans were left out of the data.

Longer thumb means bigger brains

Long thumbs allow a precise grip. Picking up a nut, holding a stick, or shaping a stone requires control. The study shows that as this skill sharpened, brain size followed.

Dexterity and intelligence were not two unrelated steps. They moved forward side by side.

“We’ve always known that our big brains and nimble fingers set us apart, but now we can see they didn’t evolve separately,” said Dr. Joanna Baker, lead author from the University of Reading.

“As our ancestors got better at picking up and manipulating objects, their brains had to grow to handle these new skills. These abilities have been fine-tuned through millions of years of brain evolution.”

Surprising connection in the brain

The researchers first assumed that thumb length would be tied to the cerebellum, since that region directs movement and coordination.

To their surprise, the connection was with the neocortex, the part of the brain responsible for thought, awareness, and sensory processing.

This shows that the thumb’s evolution was not limited to better movement but was also linked to higher cognitive demands.

Controlling fine hand actions requires more than quick reflexes. It involves planning each step, adjusting based on feedback, and maintaining focus.

These are exactly the kinds of tasks managed by the neocortex, explaining why its growth paralleled the rise of dexterous hands.

Dexterity beyond thumb length

The researchers also tested whether other hand features mattered. They found that dexterity comes from many factors including finger proportions, joint structure, and even the role of the fifth digit.

Still, thumb length proved a reliable indicator of how well a primate could manipulate objects.

The team measured “manipulation workspace” – the range where the thumb and finger can move an object. Species with longer thumbs had larger workspaces.

Those workspaces also lined up with brain size. Both hands and brains paid the cost of better skills.

Hominins in focus

Most hominins fit the general pattern, but there were exceptions. Australopithecus sediba had an unusually long thumb compared to its brain size. That didn’t make it more skilled.

In fact, biomechanical models suggest the species wasn’t great at thumb opposition. A long thumb without extra brain power did not guarantee complex manipulation.

Homo naledi also had long thumbs and smaller brains, yet it followed the general rule across primates. The difference between species shows that hands cannot be studied alone. The brain always enters the picture.

What grew inside

When researchers examined specific brain regions, the neocortex emerged as the key player. Thumb length showed a strong connection to this region, but not to the cerebellum, which mainly handles coordination.

This finding means that fine hand control relied more on higher-level processing than simple movement control.

Areas in the motor and parietal cortices likely expanded to support the growing demands of precise manipulation.

Some scientists suggest this expansion had wider effects, linking hand use with the evolution of language.

Both tool use and speech require planning, sequencing, and complex control, so shared brain networks may have supported the rise of these defining human skills.

Strongest thumb and brain link

Humans sit at the far edge of this evolutionary story. Our species combines the longest thumbs with the largest brains among primates, a pairing that gave us the capacity to make tools, form cultures, and build societies.

But this link between dexterity and intelligence did not appear suddenly with us. It runs deep across the primate family tree, shaping species long before humans emerged.

The evidence shows that smarter brains and skilled hands advanced together over millions of years. Our ability to think was never separate from our ability to grasp and shape objects.

Intelligence, in this sense, is tied to action. The human mind grew stronger because our hands demanded it.

The study is published in the journal Communications Biology.

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