Summary: A new evolutionary study provides a persuasive explanation for one of anthropology’s longest-standing puzzles—why roughly 90% of people prefer their right hand. Using comparative phylogenetic modelling across 41 primate species, researchers show that human handedness is a predictable result of two defining evolutionary changes: habitual bipedalism and substantial brain expansion.
Key Facts
- Gradual evolutionary shift: Hand preference evolved as a graded trend. Early hominins such as Ardipithecus and Australopithecus likely showed only a mild rightward bias, comparable to other great apes, while members of the genus Homo (including H. erectus and Neanderthals) show progressively stronger right-side bias, culminating in modern Homo sapiens.
- Two-stage mechanism: The researchers propose a two-step process: upright walking first freed the hands from locomotion and allowed more specialized manual behavior; later, rapid brain enlargement and reorganization solidified strong, population-level right-handedness.
- Notable exception: Homo floresiensis, the small-bodied, small-brained Indonesian hominin, is predicted to have a much weaker right-hand bias, consistent with its reduced brain size and a locomotor repertoire that included significant climbing.
- Unified comparative test: This work is the first to evaluate multiple major hypotheses for primate handedness—tool use, diet, habitat, social structure, body mass, locomotion, and brain size—within a single phylogenetic framework.
Source: Oxford University
The overwhelming dominance of the right hand in humans—present across cultures and age groups and unmatched among other primates—has puzzled researchers for decades. Despite progress in genetics, neurobiology, and development, the evolutionary origin of this consistent population-level lateralization remained unclear.
In new research led by Dr Thomas A. Püschel and Rachel M. Hurwitz (University of Oxford) with Chris Venditti (University of Reading) and published in PLOS Biology, the authors analyzed data from 2,025 individuals spanning 41 species of monkeys and apes. They applied Bayesian phylogenetic comparative methods to test longstanding explanations for handedness direction (mean handedness index, MHI) and strength (mean absolute handedness index, MABSHI).
Initial models showed humans as a clear outlier compared to other primates. However, when two variables were included—brain size (endocranial volume) and the intermembral index (a standard metric reflecting relative arm and leg lengths and thus bipedal propensity)—human exceptionalism vanished. In other words, once upright walking and larger brain size are accounted for, human handedness fits the broader evolutionary pattern found across primates.
The modelling also allowed the team to estimate handedness in extinct hominins. Their reconstruction depicts a gradual strengthening of lateralization: early hominins had modest rightward tendencies, whereas later members of the genus Homo show markedly greater right-side bias, with H. sapiens displaying the most extreme pattern documented.
A notable exception in the fossil record is Homo floresiensis. With a comparatively small brain and anatomy suggestive of mixed arboreal and upright locomotion, this species fits the model’s expectation of a weaker rightward preference, underscoring the link between locomotor behavior, brain size, and lateralized manual function.
Taken together, the findings support a two-stage evolutionary scenario. First, the shift to habitual bipedalism freed the hands for manipulation and introduced selective pressures for more specialized, lateralized manual skills. Second, subsequent neuroanatomical expansion and reorganization strengthened and stabilized a right-side dominance across human populations.
Dr Thomas A. Püschel, Wendy James Associate Professor in Evolutionary Anthropology at the University of Oxford, said: “This is the first study to test several of the major hypotheses for human handedness in a single comparative framework. Our results indicate that walking upright and increasing brain size are central to why humans show such a pronounced rightward bias. By comparing many primate species we can distinguish ancient, shared aspects of handedness from what is uniquely human.”
Key Questions Answered:
A: The comparative analysis indicates that habitual bipedalism and large brain size together explain the human pattern. Standing and walking on two legs freed the hands to specialize, and later brain expansion reinforced strong, population-level lateralization toward the right.
A: The species combined a small brain with anatomical traits suited to both climbing and some upright walking. Lacking the full suite of traits—especially large brain size and exclusive bipedalism—its predicted right-hand bias is much weaker, consistent with the model’s predictions.
A: Not entirely. The models explain why the majority shifted to the right, but they do not fully resolve why a stable minority of left-handed individuals persists. The authors highlight future research directions, including cultural factors that may stabilize handedness and ecological or genetic mechanisms that maintain left-handedness at low frequencies.
Editorial Notes:
- This article was edited by a Neuroscience News editor.
- The journal paper was reviewed in full for accuracy.
- Additional explanatory context was provided by editorial staff.
About this handedness and evolutionary neuroscience research news
Author: Lizzie Dunthorne
Source: University of Oxford
Contact: Lizzie Dunthorne – University of Oxford
Image: Image credit: Neuroscience News
Original Research: Open access. “Bipedalism and brain expansion explain human handedness” by Püschel, T. A., Hurwitz, R. M., Venditti, C. — PLOS Biology. DOI: 10.1371/journal.pbio.3003771
Abstract
Bipedalism and brain expansion explain human handedness
Humans show an unusually strong, near-universal preference for the right hand, a lateralization pattern not observed at the same scale in other primates. To investigate its origins, the authors combined phylogenetic comparative methods with meta-analysis of manual lateralization across 41 anthropoid species (n = 2,025), testing multiple eco-evolutionary hypotheses for handedness direction and strength.
The models found a measurable phylogenetic signal in both handedness direction and strength and identified modern humans as an evolutionary outlier. Crucially, inclusion of brain size (endocranial volume) and intermembral index in the models removed that outlier status, indicating these factors are central to the emergence of human handedness. The analysis further indicates that strong handedness (high MABSHI) evolved early in hominin evolution, while a pronounced directional bias (high MHI) rose with the appearance of the genus Homo.
These results support bipedalism and neuroanatomical expansion as key drivers of uniquely human lateralization and provide a comparative framework for separating human-specific adaptations from broader primate trends in the evolution of behavioral asymmetries.