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Scientists may have uncovered a deeper evolutionary explanation for why nearly 90% of humans are right-handed, and the answer could date back millions of years to when early human ancestors first began walking on two legs.
A new study from the University of Oxford suggests human handedness was not caused by a single genetic mutation or cultural habit. Instead, researchers argue it emerged gradually through two major evolutionary shifts: bipedal walking and the expansion of the human brain, according to the study published in PLOS Biology.
To investigate the origins of handedness, researchers analyzed data from 2,025 individuals across 41 species of monkeys and apes using Bayesian evolutionary models. Humans stood apart from nearly every other primate species, showing a strong population-wide preference for the right hand, while most primates displayed little consistent bias toward either side.
The researchers found that the gap became easier to explain after accounting for two biological traits: brain size and the intermembral index, a measurement comparing arm and leg length. Humans evolved relatively longer legs and shorter arms as part of adapting to upright walking, changes that appear closely linked to the development of stronger hand specialization.
The study proposes a two-stage evolutionary pathway behind modern handedness. The first phase likely began when bipedalism freed the hands from locomotion, allowing them to take on more specialized roles involving carrying, gesturing, and eventually tool use. Researchers noted that movement patterns already influence hand preference in some tree-dwelling primates that rely heavily on coordinated limb motion.
The second phase appears to have arrived later as the human brain rapidly expanded. Using evolutionary estimates, the researchers suggest species such as Australopithecus may have shown only weak right-hand preferences, while stronger handedness likely emerged in later hominins including Homo erectus, Neanderthals, and eventually modern Homo sapiens.
One intriguing outlier may have been Homo floresiensis, often referred to as the “hobbit” species because of its small body and brain size. Researchers predict it probably retained weaker handedness due to preserving more climbing-related adaptations alongside upright walking.
The findings challenge long-running theories that framed handedness primarily around language, social behavior, or toolmaking alone. Instead, the study suggests hand dominance may be deeply tied to the broader evolution of human movement and cognition.
The research also raises fresh questions about why left-handedness persisted through human evolution despite the overwhelming dominance of right-handedness today. Scientists say future studies could explore whether similar evolutionary pressures shaped limb preferences in entirely different animals, including parrots and kangaroos, potentially revealing that handedness-like behavior evolved independently across multiple species.