Why do humans lean so heavily to the right? A new study says the answer may trace back to two big evolutionary shifts, walking upright and growing much larger brains.
Researchers led by the University of Oxford found humans may have become overwhelmingly right-handed as those two traits developed over time. The study, published in PLOS Biology, suggests a mild hand preference in early ancestors later became the extreme right-hand dominance seen in modern humans.
Around 90 percent of people across cultures prefer using their right hand, a level of dominance the researchers said is not seen in any other primate species.
The research was conducted by Dr Thomas A. Püschel and Rachel M. Hurwitz from Oxford’s School of Anthropology and Museum Ethnography, together with Professor Chris Venditti from the University of Reading.
The team analysed data from 2,025 monkeys and apes across 41 primate species. Using Bayesian modelling that accounted for how species are evolutionarily related, they tested several theories about the origins of handedness.
They examined tool use, diet, habitat, body size, social structure, brain size and movement patterns.
Humans initially appeared to sit apart from every other primate in the analysis. But that changed after researchers added two traits into their models, brain size and the ratio between arm length and leg length, which they used as a marker of bipedal movement.
After accounting for those features, humans no longer looked like such an evolutionary exception.
The findings suggest upright walking and larger brains together may explain why humans developed such a strong preference for the right hand.
The study also allowed the researchers to estimate likely handedness in extinct human ancestors.
They found early hominins such as Ardipithecus and Australopithecus probably had only mild right-hand preferences, similar to modern great apes today.
That pattern appears to strengthen with the emergence of the genus Homo. Species including Homo ergaster, Homo erectus and Neanderthals likely had increasingly strong right-hand preferences, leading to the extreme dominance seen in modern humans.
One species did not fit the pattern neatly, Homo floresiensis, often nicknamed the “hobbit”. Researchers predicted it had a much weaker right-hand bias.
According to the team, that fits the broader pattern because Homo floresiensis had a relatively small brain and retained physical adaptations for both climbing and upright walking, rather than being fully specialised for bipedal movement.
The researchers said the evidence points to a two-stage evolutionary process. First, upright walking freed the hands from locomotion, creating pressures that favoured more specialised and asymmetric hand use. Later, as human brains became larger and more complex, the preference for the right hand grew much stronger and more widespread.
Dr Thomas A. Püschel, Wendy James Associate Professor in Evolutionary Anthropology at the University of Oxford, said: “This is the first study to test several of the major hypotheses for human handedness in a single framework. Our results suggest it is probably tied to some of the key features that make us human, especially walking upright and the evolution of larger brains. By looking across many primate species, we can begin to understand which aspects of handedness are ancient and shared, and which are uniquely human.”
The study also raises questions about why left-handedness has persisted through human evolution and how human culture may have reinforced right-handedness over time.
The paper is titled Bipedalism and brain expansion explain human handedness.
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