Summary: A new evolutionary study offers a compelling solution to one of anthropology’s oldest puzzles: why roughly 90% of humans are right-handed, a population-level bias unique among primates.
The team used Bayesian modeling across 41 primate species to test multiple evolutionary hypotheses. The findings reveal that human handedness is not an inexplicable anomaly, but rather the direct mathematical consequence of two defining human traits: walking on two legs (bipedalism) and dramatic brain expansion.
Key Facts
- The Evolutionary Trajectory: Handedness developed as a gradient; early hominins like Ardipithecus and Australopithecus possessed only a mild rightward bias, which significantly hardened through Homo erectus and Neanderthals before peaking in Homo sapiens.
- The Two-Stage Formula: Evolution split the process into two steps: upright walking first freed the hands from locomotion to allow manual specialization, while later brain expansion and reorganization permanently locked in the near-universal rightward extreme.
- The “Hobbit” Exception: Homo floresiensis, the small-brained extinct hominin from Indonesia, presents a striking exception with a much weaker predicted right-hand preference, matching its small brain and partial climbing anatomy.
- A Unified Framework: This research marks the first time major competing theories of handedness, including tool use, diet, habitat, and social structure, were tested simultaneously within a single comparative framework.
Source: Oxford University
It is one of the strangest puzzles in human evolution. About 90% of people across every human culture favour their right hand – with no other primate species showing a population-level preference on this scale.
Despite decades of research into the brains, genes and development behind handedness, why humans ended up so overwhelmingly right-handed has remained an evolutionary enigma.
Now, new research led by the University of Oxford, published in PLOS Biology, suggests the answer comes down to two defining features of human evolution – walking on two legs, and the dramatic expansion of the human brain.
The study, by Dr Thomas A. Püschel and Rachel M. Hurwitz at Oxford’s School of Anthropology and Museum Ethnography, with Professor Chris Venditti at the University of Reading, brought together data on 2,025 individuals across 41 species of monkeys and apes.
Using Bayesian modelling that accounts for evolutionary relationships between species, the team tested the major existing hypotheses for why handedness evolved: including tool use, diet, habitat, body mass, social organisation, brain size and locomotion.
Humans sat conspicuously outside the pattern that explained every other primate, but when the researchers added two factors into the model – brain size and the relative length of our arms versus our legs (a standard anatomical marker of bipedal locomotion) – that exceptional status disappeared. In other words, once you account for upright walking and a large brain, humans stop looking like an evolutionary anomaly.
Using the same models, the team was also able to estimate likely handedness in extinct human ancestors. The picture that emerges is a gradient; early hominins such as Ardipithecus and Australopithecus probably had only mild rightward preferences, broadly similar to modern great apes.
With the appearance of the genus Homo, the bias strengthens markedly – through Homo ergaster, Homo erectus and Neanderthals – reaching its modern extreme in Homo sapiens.
There is one striking exception: Homo floresiensis, the small-brained “hobbit” species from Indonesia, shows a much weaker predicted preference. The researchers suggest this fits the wider pattern: floresiensis had a small brain and a body adapted to a mix of upright walking and climbing, rather than full bipedalism.
The findings point to a two-stage story. Walking upright came first, freeing the hands from the work of locomotion and creating new selective pressure for fine, lateralised manual behaviours. Larger brains came later, and as they grew and reorganised, the rightward bias hardened into the near-universal pattern seen today.
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 framework.
“Our results suggest it is probably tied to some of the key features that make us human, especially walking upright and the evolution of larger brains. By looking across many primate species, we can begin to understand which aspects of handedness are ancient and shared, and which are uniquely human.’
The study leaves open questions for future research, including the role of cumulative human culture in stabilising right-handedness, why left-handedness has persisted at all, and whether similar patterns of limb preference seen in animals such as parrots and kangaroos point to a deeper, convergent story across the wider animal kingdom.
Key Questions Answered:
A: It comes down to walking on two legs and having massive brains. When our ancestors stood up, their hands were freed from walking, creating intense evolutionary pressure to specialize. As our brains grew and structurally reorganized over millions of years, that manual specialization hardened into a universal right-side dominance.
A: Homo floresiensis had a very small brain and an anatomy still partially built for climbing trees instead of pure upright walking. Because they lacked both full bipedalism and a large brain, their predicted rightward bias remained weak, perfectly proving the study’s model.
A: Not completely. While the model solves why the majority shifted right, it leaves open questions for future research regarding why left-handedness has consistently persisted at a steady minority rate, and what role cultural stabilization plays in keeping the 90/10 split.
Editorial Notes:
- This article was edited by a Neuroscience News editor.
- Journal paper reviewed in full.
- Additional context added by our staff.
About this handedness and evolutionary neuroscience research news
Author: Lizzie Dunthorne
Source: University of Oxford
Contact: Lizzie Dunthorne – University of Oxford
Image: The image is credited to 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 exhibit a striking and near-universal population-level right-hand preference, an evolutionary singularity unmatched among primates. Despite its pervasiveness, the origins of this lateralization remain poorly understood.
Here, we combine phylogenetic comparative methods with meta-analysis to investigate manual lateralization across 41 anthropoid species (n = 2,025), testing longstanding eco-evolutionary hypotheses for handedness direction (mean handedness index, MHI) and strength (mean absolute handedness index, MABSHI).
Our models reveal significant phylogenetic signal for both traits and identify Homo sapiens as an evolutionary outlier, exhibiting exceptional rightward bias and strength relative to phylogenetic expectations.
However, this outlier status disappears when brain size (endocranial volume) and intermembral index are included, suggesting these factors are central to the emergence of human handedness. We also show that high MABSHI evolved early in hominin evolution, while MHI increased to unparalleled levels with the appearance of the genus Homo.
Our findings identify bipedalism and neuroanatomical expansion as likely key drivers of uniquely human lateralization, while also revealing broader ecological patterns shaping handedness across primates.
This work provides a framework for disentangling human-specific adaptations from general primate trends in the evolution of behavioral asymmetries.
