Roughly 9 in 10 people alive today are right-handed — a bias so lopsided it sets our species apart from every other primate on Earth. Apes and monkeys show no such extreme preference, yet humans have carried this quirk across every culture and continent for as far back as the fossil record reaches.
Scientists have puzzled over why for decades. Now, new research published in PLOS Biology suggests the answer may lie in two unexpected changes deep in our evolutionary past — changes that began reshaping the human body long before Homo sapiens ever existed.
A species unlike any other primate
Humans are the only primate species with such an overwhelming population-wide preference for one hand. Roughly 90% of people worldwide default to their right hand for writing, throwing, eating, and dozens of other tasks. Other apes and monkeys do show individual hand preferences — a particular chimpanzee might reliably reach with its left — but no other primate species comes close to our lopsided, species-wide bias.
Scientists have proposed many competing explanations: diet, social organization, habitat, and tool use have all had their advocates. Each hypothesis carried some intuitive appeal. None had been tested rigorously across a wide range of species at once.
The new study in PLOS Biology set out to change that. Researchers used evolutionary and statistical methods to analyze handedness data across 41 different primate species, systematically testing the leading hypotheses side by side. The results pointed clearly toward two factors that the others couldn’t match.
Two factors rise above the rest
Of all the candidate explanations examined, two emerged as the most likely drivers of human right-handedness: brain size, and the relative length of arms to legs — a proxy researchers used to measure the shift toward bipedal locomotion.
The logic behind bipedalism is fairly intuitive. Once our ancestors began walking upright, their hands were no longer needed for movement, and that freedom created evolutionary pressure to specialize. Individuals who could perform fine motor tasks more reliably with one hand may have had a survival and reproductive advantage — a marginal edge that compounded over generations.
Brain size and reorganization reinforced the process in a more specific way. While bipedalism may have pushed hand preferences to develop initially, the researchers suggest the brain’s expansion is more closely tied to which hand became dominant. As they write in the paper, “our unusual gait was the main initial driver of our exceptional handedness strength,” with the larger brain “more linked to the directionality.”
Walking upright may have started the process. A bigger, more lateralized brain appears to have locked in the right side.
Tracing the bias through our extinct ancestors
To test whether this evolutionary sequence held up historically, the researchers estimated likely handedness across a range of extinct hominin species — and the pattern they found was consistent with a gradual intensification over time.
Early hominins like Ardipithecus ramidus and Australopithecus afarensis showed only mild right-hand preferences in the researchers’ estimates. The bias strengthened through Homo erectus and Homo neanderthalensis before reaching its current peak in Homo sapiens.
One species stood out as an exception: Homo floresiensis, the small-brained hominin discovered on the Indonesian island of Flores. The researchers predicted a weaker right-hand preference for this species. Its combination of a relatively small brain and a locomotion style that blended upright walking with climbing may explain the divergence.
So why is anyone still left-handed?
If right-handedness is so deeply embedded in human biology, the persistence of left-handedness is itself a puzzle worth taking seriously. Around 10% of the global population is left-handed — a minority that’s remained remarkably stable across cultures and time periods. Left-handed people typically have a dominant right hemisphere, the mirror image of the pattern seen in right-handers.
Genetics appears to play a role. In 2024, scientists identified rare variants of a gene called TUBB4B as more common among left-handed individuals. But genetics may not be the whole story. Clyde Francks, a geneticist at the Max Planck Institute for Psycholinguistics, has suggested that some people become left-handed “simply due to random variation during development of the embryonic brain, without specific genetic or environmental influences.” The two explanations aren’t mutually exclusive — left-handedness may have multiple routes to the same outcome.
Bipedalism’s reach beyond humans
The connection between upright posture and limb preference may not be a uniquely human story. Researchers see bipedalism as a broader evolutionary trigger, one that appears to have shaped handedness in other species as well.
Many kangaroo and wallaby species show a strong preference for their left paw when gathering food or grooming, and scientists suspect this too relates to their upright stance freeing up their forelimbs. Most parrot species show consistent side preferences for both eye and foot when handling food, suggesting a similar kind of lateralization at work.
“It seems like bipedalism is a triggering factor that pushes forward the evolution of handedness,” biologist Yegor Malashichev has said. “Standing on your hind legs frees up your forelimbs, and you can do with them what you like.”
That framing invites a broader reflection. A trait as intimate and personal as which hand you write with may ultimately trace back not to culture, habit, or even genetics alone — but to the moment, millions of years ago, when our ancestors first stood up and walked. The body changed, the brain followed, and the hand we favor today may be one of the quietest echoes of that ancient shift.