Why are so many people right-handed?

A new study suggests the answer may trace back to two major shifts in human evolution: walking upright and growing bigger brains.

According to Sci.News, researchers said those changes may help explain why roughly 90% of humans favor their right hand today.

Scientists from the University of Oxford and the University of Reading examined handedness data from 2,025 primates across 41 monkey and ape species to test why humans are such a striking outlier among primates.

They examined several leading explanations for handedness, including tool use, diet, habitat, body mass, social organization, brain size, and locomotion.

On their own, those factors could explain patterns in other primates — but not humans.

That changed when the researchers added two traits to their model: brain size and the ratio of arm length to leg length, a standard anatomical cue for bipedalism.

Once those two factors were taken into account, humans no longer appeared evolutionarily anomalous. The findings, published in PLoS Biology, suggest that upright walking and larger brains together may have driven the unusually strong right-hand preference seen in our species.

The team also used the same model to estimate handedness in extinct human relatives, finding that early hominins likely had only mild rightward preferences, while later members of the genus Homo showed a much stronger bias.

Handedness is noticed in everyday tasks such as writing, cooking, and using tools. This research suggests it may reflect much bigger evolutionary changes in how humans moved and how their brains developed.

The findings also help explain why humans differ so sharply from other primates. While apes and monkeys can show hand preferences, strong manual lateralization — where one hand is clearly favored and ambidexterity is rare — appears far more pronounced in humans.

Rather than pointing to a sudden shift, the study suggests a gradual evolutionary process. 

Researchers said early upright walking may have freed the hands from locomotion, opening the door for more specialized hand use.

Later, as brains expanded and reorganized, that preference appears to have become much more fixed, eventually reaching the extreme seen in modern Homo sapiens.

One exception stood out. Homo floresiensis, the small-brained human relative from Indonesia, was predicted to have a much weaker right-hand preference, reinforcing the idea that brain size played a major role.

Researchers are using evolutionary models to answer long-standing questions about what makes humans distinct.

By comparing living primates and then applying those patterns to extinct species, the team was able to build a more detailed picture of how handedness may have changed across the human lineage.

That kind of work may help scientists better understand the connections among anatomy, movement, and cognition.

Traits that seem ordinary, such as reaching first with one hand, may offer clues to a long evolutionary story.

Studies like this may also sharpen scientists' thinking about human brain development and the physical changes that shaped our ancestors' everyday lives.

As new fossil evidence and comparative data emerge, researchers may be able to refine the timeline further and test whether other uniquely human traits followed a similar pattern.

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