About nine out of every ten humans on Earth are righties. This strange imbalance stretches across cultures, continents, and history itself. No other primate species comes remotely close to showing such a strong population-wide preference. 

Scientists have spent decades trying to explain it through brain structure, genetics, tool use, or even culture, yet the mystery has stubbornly survived. 

Now, a new study from researchers at the University of Oxford suggests the answer may lie in two evolutionary changes that transformed our species more than any others: walking upright and growing unusually large brains. 

The study authors argue that humans became strongly right-handed through a gradual evolutionary process tied to bipedalism and brain expansion rather than a single genetic switch.

“This is the first study to test several of the major hypotheses for human handedness in a single framework,” Thomas A. Püschel, one of the study authors and a professor of anthropology at the University of Oxford, said.

Humans looked evolutionarily weird

The research team analyzed handedness data from 2,025 individuals across 41 species of monkeys and apes. Using Bayesian evolutionary models, they tested many major explanations for handedness, including diet, tool use, social structure, habitat, body size, and brain size.

The study measured handedness using something called the Mean Handedness Index (MHI), where positive numbers indicate right-hand preference. Humans scored 0.76 — dramatically higher than other primates, most of which clustered near zero, meaning they showed little consistent population-wide preference. 

“Humans display a pronounced right-handed bias (MHI = 0.76), which contrasts sharply with the phylogenetic prediction of the reduced model excluding humans (MHI = 0.0),” the study authors note.

Humans were the only species with a strong and statistically credible rightward bias. However,  the picture changed when researchers added two specific variables into their models: brain size and the ‘intermembral index,’ which compares arm length to leg length. 

Humans have unusually long legs compared with arms, a hallmark of bipedal walking. Once those two factors were included, humans no longer looked evolutionarily exceptional. 

Walking upright may have freed the hands

The researchers propose a two-stage explanation for human handedness. First came bipedalism. As early human ancestors began walking upright, their hands were freed from locomotion. 

This likely created new evolutionary pressure for specialized hand use during tasks such as carrying objects, manipulating tools, or gesturing. The study found that locomotion strongly influences handedness patterns across primates. 

For instance, tree-dwelling species often show stronger hand preferences because moving through branches requires precise and coordinated movements. Humans may have taken that pattern in a different direction. Upright walking allowed increasingly specialized use of one hand over the other.

The second phase came later with brain expansion

Using evolutionary models, the researchers estimated handedness in extinct human relatives. Early hominins such as Ardipithecus and Australopithecus likely had weak right-hand preferences similar to modern apes. 

However, the bias strengthened in species such as Homo erectus and Neanderthals before reaching its modern extreme in Homo sapiens. “It is with the emergence of the genus Homo, and particularly the onset of significant encephalization, that we observe a marked increase in MHI values,” the study authors added.

One unusual exception was Homo floresiensis, the small-brained ‘hobbit’ species discovered in Indonesia. The models predicted much weaker handedness in this species, possibly because it retained adaptations for both climbing and upright walking.

The significance of these findings

The study suggests human right-handedness may be deeply rooted in the same evolutionary changes that transformed how our ancestors moved and interacted with the world. 

However, some important questions remain unanswered. For instance, scientists still do not know why left-handedness survived throughout human evolution, or how much human culture helped strengthen right-hand dominance over time.

“Humans are unique in displaying cumulative cultural evolution, which may amplify or stabilize behavioral asymmetries,” the study authors note.

The researchers also say future studies could investigate whether limb preferences in animals such as parrots or kangaroos evolved through similar evolutionary pressures, potentially revealing that handedness-like behavior emerged independently across different branches of the animal kingdom.

The study is published in the journal PLOS Biology.