Summary: A new evolutionary study offers a clear and testable explanation for one of anthropology’s longest-standing puzzles: why about 90% of people worldwide favour their right hand. This population-level bias is unique among primates, and the research shows it follows logically from two key human traits—bipedalism and dramatic brain expansion.
Key Facts
- The evolutionary trajectory: Hand preference emerged gradually. Early hominins such as Ardipithecus and Australopithecus likely displayed only a weak rightward bias. That bias intensified through members of the genus Homo—including Homo ergaster, Homo erectus, and Neanderthals—culminating in the pronounced right-hand dominance of modern Homo sapiens.
- Two-stage process: The evolution of upright walking first released the hands from locomotor duties, creating opportunities for manual specialization. Later, large-scale brain enlargement and cortical reorganization reinforced and consolidated strong right-side lateralization.
- The “Hobbit” exception: Homo floresiensis appears as an exception: its small brain and partially climbing-adapted anatomy predict a much weaker population-level right-hand bias compared with modern humans, consistent with the study’s model.
- A unified comparative test: For the first time, the major hypotheses about handedness—tool use, diet, habitat, social structure, body mass, locomotion and brain size—were evaluated together across multiple primate species using phylogenetically informed statistical models.
Source: University of Oxford
Overview
About 90% of people across cultures prefer their right hand, a scale of population-level lateralization unmatched in other primates. Despite extensive research into the genetics, development and neurobiology of handedness, the ultimate evolutionary drivers of this strong rightward bias have remained uncertain.
A new comparative study led by researchers at the University of Oxford and the University of Reading re-evaluates this question using phylogenetic Bayesian models. The team combined data from 2,025 individuals spanning 41 species of monkeys and apes, then tested multiple long-standing eco-evolutionary hypotheses to identify which traits best explain differences in handedness direction and strength across primates.
When standard ecological and behavioural predictors were used alone, humans stood out as a clear outlier. However, adding two biologically meaningful variables—brain size (endocranial volume) and the intermembral index (a proxy for the degree of bipedal locomotion)—removed that exceptional status. In other words, once upright walking and enlarged brains are taken into account, humans fit into the broader primate pattern rather than being inexplicably anomalous.
The models also allowed the researchers to infer handedness in extinct hominin species. The reconstructed pattern is gradual: mild rightward tendencies in the earliest hominins gave way to stronger lateralization with the emergence of the genus Homo, and the strongest, near-universal right-hand preference appears in modern humans. The notable exception—Homo floresiensis—aligns with the model’s expectation because of its small brain and mixed locomotor anatomy.
Together the results support a two-stage hypothesis for the evolution of human handedness. Stage one: bipedalism freed the hands and created selective pressures favouring manual specialization and lateralized manual behaviours. Stage two: substantial brain expansion and reorganization amplified and stabilized a population-level bias toward the right hand.
These findings are important for both evolutionary biology and neuroscience because they provide a comparative framework that separates broad primate trends from uniquely human adaptations, linking behavioral asymmetries to measurable anatomical and neuroanatomical changes.
Key Questions Answered:
A: The study indicates this pattern arises from the combination of upright walking and large brains. Bipedalism freed the hands for specialized tasks, and subsequent brain enlargement and reorganization reinforced lateralized manual control, producing the strong rightward bias seen in humans.
A: Homo floresiensis had a comparatively small brain and anatomical features indicating a mix of upright walking and climbing. Because it lacked both full bipedal specialization and a large brain, the model predicts a much weaker population-level right-hand bias, consistent with the fossil evidence.
A: Not entirely. The research explains why the majority shifted toward right-handedness but leaves open why a minority of left-handers persists. Future work is needed to explore mechanisms such as social and cultural stabilization, frequency-dependent selection, or balancing selection that might maintain left-handedness at low but steady levels.
Editorial Notes:
- This article was edited by a Neuroscience News editor.
- The underlying journal paper was reviewed in full by editorial staff.
- Additional context and clarification were added by the news team for readability.
About this handedness and evolutionary neuroscience research news
Author: Lizzie Dunthorne
Source: University of Oxford
Contact: Lizzie Dunthorne – University of Oxford
Image credit: The image is credited to Neuroscience News
Original research: Open access. Title: “Bipedalism and brain expansion explain human handedness” by Püschel, T. A., Hurwitz, R. M., Venditti, C., published in PLOS Biology. DOI: 10.1371/journal.pbio.3003771
Abstract
Bipedalism and brain expansion explain human handedness
Humans display a pronounced population-level preference for the right hand, a degree of lateralization without parallel in other primates. To investigate its origins, the authors combined phylogenetic comparative methods and meta-analysis to assess manual lateralization across 41 anthropoid species (n = 2,025). The study tested multiple eco-evolutionary hypotheses about handedness direction (mean handedness index, MHI) and strength (mean absolute handedness index, MABSHI).
The analyses detected significant phylogenetic signal for both handedness traits and identified modern humans as an outlier with exceptional rightward bias and strength relative to phylogenetic expectations. Crucially, when brain size (endocranial volume) and the intermembral index were incorporated, human exceptionalism vanished, indicating these variables are central to the emergence of human handedness. The results also show that high strength of handedness evolved early among hominins, while directional bias increased markedly with the origin of the genus Homo.
Overall, the findings highlight bipedalism and neuroanatomical expansion as likely key drivers of uniquely human lateralization and provide a comparative framework to distinguish human-specific adaptations from broader primate trends in the evolution of behavioral asymmetries.