Science discovers why 90% of humanity is right-handed: the key may lie in the way we walk

Approximately 90% of people across all human cultures are right-handed, and no other primate species shows a population-wide preference of this magnitude. But why?

New research led by the University of Oxford and published in PLOS Biology suggests that the answer comes down to two defining features of human evolution: walking on two legs and the dramatic expansion of the human brain.

The Oxford research team gathered data from 2,025 individuals across 41 species of monkeys and apes to test the leading hypotheses about the evolution of handedness. In other words, they investigated why humans tend to consistently favor organs and limbs located on one side of the body over the other.

Humans: an evolutionary exception

The researchers examined every possible scenario, taking into account variables such as tool use, diet, habitat, body mass, social organization, brain size, and locomotion. Yet in every case, humans remained an evolutionary exception, since no other ape demonstrates such a strong preference for a single limb.

Brain size and limb proportions

This exception only disappeared when the researchers at the University of Oxford added two factors to their model: brain size and the proportion of arm length to leg length. This is a key factor in explaining bipedalism, meaning why humans eventually began walking exclusively on two limbs.

The study therefore concluded that larger brains and longer legs are closely linked to the use of a dominant hand.

The evolutionary explanation

The upper limbs became free with the adoption of a bipedal, upright posture, which increased the ability to manipulate objects and communicate through gestures. As a result, a new selective pressure emerged for fine, lateralized hand movements.

Later, as the cerebral cortex expanded, the bias toward right-handedness became firmly established until it evolved into the nearly universal pattern we observe today.

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