Why Is Nearly Every Person Right-Handed—But Not Every Ape and Monkey? New Research Explores the Evolutionary Origins of Human Handedness
Brain size and bipedalism are the most likely drivers of our species’ right-hand dominance, according to new research
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The vast majority of people alive today use their right hand for everything from writing and throwing to eating to brushing their teeth. But why? Humans are the only primate species with such overwhelming right-hand dominance, and scientists have long puzzled over the evolutionary origins of this trend.
New research, published in the journal PLOS Biology, suggests human right-handedness may be related to the development of our larger brains and the shift to walking upright on two legs.
Scientists reached this conclusion after using evolutionary and statistical methods to analyze handedness across 41 different primate species. They probed several of the leading explanations for the evolution of human right-handedness, such as diet, social organization, tool use and habitat. Two factors emerged as the most likely drivers: brain size and the relative length of our arms to our legs, a proxy for bipedal locomotion.
Based on the findings, researchers suspect human right-handedness evolved gradually over time. First, humans started walking on two legs, which meant their hands were available for other tasks. Over time, humans who were better at performing fine motor tasks with one hand probably became more likely to survive and reproduce. This was followed by a size increase and reorganization of the brain, which cemented the propensity for using our right hands, the researchers posit.
“Our unusual gait was the main initial driver of our exceptional handedness strength,” the researchers write in the paper, “with our large brain more linked to the directionality.”
Did you know? The origins of bipedalism
Scientists have long debated why human ancestors evolved to walk upright. One theory posits that human ancestors began walking upright when forests retreated and hominins began spending more time on the ground, traveling and foraging for food. Another theory proposes that human ancestors began walking upright to move around tree branches in search of food.
Researchers also estimated the likely handedness of our extinct ancestors, which further supported this theory. Their investigation suggests early hominins, like Ardipithecus ramidus and Australopithecus afarensis, had mild preferences for right-handedness. The bias got stronger with subsequent species, including Homo erectus and Homo neanderthalensis, before reaching its peak with Homo sapiens.
Homo floresiensis was the exception to this pattern, with a weaker predicted preference for right-handedness. The researchers suspect this species may be an outlier because it had a relatively small brain and performed a blend of upright walking and climbing.
Though the findings hint at the evolutionary origins of human right-handedness, they also raise additional questions. For instance, what role did culture play in establishing right-hand dominance among humans? And, if right-handedness is so deeply rooted, why is anyone left-handed anymore?
Around 10 percent of the population is left-handed, a tendency that researchers believe stems from brain asymmetry that develops before birth. Left-handed people tend to have dominant right sides of their brain, while right-handed people tend to have dominant left sides.
In 2024, scientists discovered that rare variants of a specific gene called are more common in left-handed people, suggesting genetics might play a role in whether a person is left-handed or right-handed. However, researchers also suspect some people become left-handed “simply due to random variation during development of the embryonic brain, without specific genetic or environmental influences,” Clyde Francks, a geneticist at the Max Planck Institute for Psycholinguistics, told Reuters’ Will Dunham.
Researchers are also curious to know whether there are any links between the human evolution of right-handedness and similar limb preference patterns in other animals.
Most parrot species, for instance, show a strong preference for one side of their body or the other. When eating food, they either like to use their left eye to ogle it and their left foot to pick it up, or their right eye and foot.
Similarly, many species of kangaroo and wallaby also show strong limb preferences, with most preferring to use their left paw for everyday tasks like gathering food and grooming. As with humans, researchers suspect the marsupials’ preferences might be related to their upright posture.
“It seems like bipedalism is a triggering factor that pushes forward the evolution of handedness,” the late Yegor Malashichev, then a biologist at Saint Petersburg State University, told Smithsonian magazine’s Devin Powell in 2015. “Standing on your hind legs frees up your forelimbs, and you can do with them what you like.”
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