It is one of the strangest puzzles in human evolution. About 90% of people across every human culture favour their right hand - with no other primate species showing a population-level preference on this scale. Despite decades of research into the brains, genes and development behind handedness, why humans ended up so overwhelmingly right-handed has remained an evolutionary enigma.
Now, new research led by the University of Oxford, published in PLOS Biology, suggests the answer comes down to two defining features of human evolution - walking on two legs, and the dramatic expansion of the human brain.
The study, by Dr Thomas A. Püschel and Rachel M. Hurwitz at Oxford's School of Anthropology and Museum Ethnography, with Professor Chris Venditti at the University of Reading, brought together data on 2,025 individuals across 41 species of monkeys and apes. Using Bayesian modelling that accounts for evolutionary relationships between species, the team tested the major existing hypotheses for why handedness evolved: including tool use, diet, habitat, body mass, social organisation, brain size and locomotion.
Humans sat conspicuously outside the pattern that explained every other primate, but when the researchers added two factors into the model - brain size and the relative length of our arms versus our legs (a standard anatomical marker of bipedal locomotion) - that exceptional status disappeared. In other words, once you account for upright walking and a large brain, humans stop looking like an evolutionary anomaly.
Using the same models, the team was also able to estimate likely handedness in extinct human ancestors. The picture that emerges is a gradient; early hominins such as Ardipithecus and Australopithecus probably had only mild rightward preferences, broadly similar to modern great apes. With the appearance of the genus Homo, the bias strengthens markedly - through Homo ergaster, Homo erectus and Neanderthals - reaching its modern extreme in Homo sapiens.
There is one striking exception: Homo floresiensis, the small-brained "hobbit" species from Indonesia, shows a much weaker predicted preference. The researchers suggest this fits the wider pattern: floresiensis had a small brain and a body adapted to a mix of upright walking and climbing, rather than full bipedalism.
The findings point to a two-stage story. Walking upright came first, freeing the hands from the work of locomotion and creating new selective pressure for fine, lateralised manual behaviours. Larger brains came later, and as they grew and reorganised, the rightward bias hardened into the near-universal pattern seen today.
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 leaves open questions for future research, including the role of cumulative human culture in stabilising right-handedness, why left-handedness has persisted at all, and whether similar patterns of limb preference seen in animals such as parrots and kangaroos point to a deeper, convergent story across the wider animal kingdom.
