Why is almost everyone right-handed? May be linked to walking upright, Study finds

For decades, scientists have struggled to explain one of the biggest mysteries in human evolution: why nearly 90 per cent of people across cultures are right-handed, while no other primate species shows such a strong population-wide preference.

A new study led by researchers at the University of Oxford suggests the answer may be tied to two major evolutionary developments that helped shape modern humans — walking on two legs and the growth of larger brains.

The research, published in PLOS Biology, analysed data from 2,025 individuals across 41 species of monkeys and apes. The study was conducted by Thomas A. Püschel and Rachel M. Hurwitz from Oxford’s School of Anthropology and Museum Ethnography, along with Chris Venditti from the University of Reading.

Using Bayesian evolutionary modelling, the researchers tested several long-standing theories behind handedness, including the influence of tool use, diet, habitat, body size, social structure, brain development and movement patterns.

Humans initially appeared to be a clear outlier compared with other primates. However, when the researchers factored in brain size and the ratio between arm and leg length — an important anatomical indicator of upright walking — humans no longer stood apart from the broader evolutionary pattern.

The findings suggest that bipedalism and brain expansion together may have driven the development of strong right-handedness in humans.

The researchers also used the model to estimate handedness patterns among extinct human ancestors. Early hominins such as Ardipithecus and Australopithecus likely showed only mild right-hand preferences, similar to those seen in modern great apes.

The tendency appears to strengthen significantly with the emergence of the genus Homo, including species such as Homo erectus and Neanderthals, before reaching the strong right-handed dominance seen in modern Homo sapiens.

One notable exception was Homo floresiensis, often referred to as the “hobbit” species. Researchers predicted that this small-brained species had a much weaker right-hand preference, possibly because it retained a mix of climbing and upright walking adaptations rather than full bipedalism.

According to the study, the evolutionary shift likely occurred in two stages. Walking upright may have first freed the hands from locomotion, encouraging specialised manual activity. Later, as human brains expanded and reorganised, right-handedness became increasingly dominant.

“This is the first study to test several of the major hypotheses for human handedness within a single framework,” said Dr Püschel. “Our findings suggest handedness is closely linked to key traits that define humans, especially upright walking and larger brains.”

The researchers said several questions still remain unanswered, including why left-handedness continues to persist in human populations and whether similar limb preferences observed in animals like parrots and kangaroos reflect a broader evolutionary pattern across species.