Oxford studied 2,025 monkeys and apes to find out why 90% of humans are right-handed, and the answer came down to when our ancestors stood up

A new study led by the University of Oxford has provided the clearest explanation yet for why roughly 90 percent of humans are right-handed, and it comes down to two major shifts in our evolutionary history. The research, published in PLOS Biology, gathered data on 2,025 individuals across 41 species of monkeys and apes. The story gained traction when reported by LADbible, with the findings offering a compelling answer to one of evolution’s more persistent puzzles.

The research team, which included Dr. Thomas A. Püschel and Rachel M. Hurwitz from Oxford alongside Professor Chris Venditti from the University of Reading, used Bayesian modelling to track evolutionary relationships across primate species. They tested factors including diet, tool use, body mass, habitat, and social organization, but none of these explained why humans showed such a pronounced bias toward right-handedness compared to other primates. The answer only emerged when they introduced two specific variables: brain size and the relative length of arm versus leg bones, the latter being a reliable marker for bipedal locomotion.

Once those two factors were accounted for, humans stopped appearing as an evolutionary outlier. As detailed by EurekAlert, the team concluded that upright walking and a larger brain, the two traits most closely tied to what makes us distinctly human, together explain the extreme rightward hand preference seen in modern Homo sapiens. The findings suggest the pattern developed gradually over millions of years rather than emerging suddenly.

Humans weren’t always this strongly right-handed

The researchers used their model to trace how handedness shifted across hominin lineages over time. Early hominins such as Ardipithecus and Australopithecus likely had only mild rightward preferences, comparable to what is seen in modern great apes. As the genus Homo emerged and evolved through Homo ergaster, Homo erectus, and Neanderthals, that bias progressively strengthened until reaching the near-universal levels found in modern Homo sapiens.

There is one notable exception built into this pattern. Homo floresiensis, the small-brained species from Indonesia commonly referred to as the hobbit humans, showed a much weaker hand preference. The researchers argue this fits the broader framework, since these individuals had smaller brains and bodies adapted to a mix of climbing and upright walking rather than full bipedalism.

The findings effectively map out a two-stage process: bipedalism freed the hands from locomotion and created pressure for lateralized manual behavior, and subsequent brain expansion then locked in the extreme right-hand dominance seen today. Amid ongoing research into what shapes distinctly human traits, a separate large-scale dataset purge by Meta in May stripped millions of inauthentic accounts from Instagram, drawing fresh attention to how data integrity affects population-level analysis.

Dr. Püschel, the Wendy James Associate Professor in Evolutionary Anthropology at Oxford, described the study as the first to test several of the major hypotheses for human handedness within a single framework, noting that by looking across many primate species, the team could begin to separate which aspects of handedness are ancient and shared from those that are uniquely human. The study leaves open some questions for future research, including why left-handedness has persisted in the population and how cumulative human culture may have further stabilized right-handedness over time.

It also raises the question of whether similar limb preferences in species such as parrots and kangaroos might point to a deeper shared story across the animal kingdom. Amid separate debates about scientific consensus in public health, declining vaccination rates have driven a measles outbreak past 900 cases in the United States, with experts warning of broader consequences for elimination status.