Study gives new clues on right-handedness

Thomas A. Püschel and colleagues have offered new evidence on a question that has long sat at the edge of human evolution: why about 90% of people are right-handed. Their study, published April 27 in PLOS Biology, compared handedness across 41 anthropoid species and concluded that two traits — upright walking and larger brains — best explain why humans became such an outlier. The work resurfaced on May 24 in a science roundup on X, where it was grouped with other recent research items. The paper does not argue that a single “right-handed gene” suddenly appeared. It argues that the human pattern emerged gradually as hominins changed how they moved and how their brains were organized. ### Why did researchers look beyond humans to explain handedness? The Oxford-led team built its analysis around 2,025 individuals from 41 species of monkeys and apes, rather than starting with humans alone. That let the researchers test whether human handedness fits broader primate patterns or stands apart from them. Dr. Thomas A. Püschel said in an Oxford release that this was the first study to test several major handedness hypotheses in a single framework. (journals.plos.org) The models examined factors including tool use, diet, habitat, body mass, social organization, brain size and locomotion, according to the university. ### What exactly did the study find? (journals.plos.org) PLOS Biology reported that Homo sapiens showed an unusually strong population-level rightward bias compared with other primates. The paper described humans as an evolutionary outlier when researchers looked at handedness direction and strength on their own. The key result came when the researchers added endocranial volume, a proxy for brain size, and intermembral index, an anatomical measure linked to bipedalism. (ox.ac.uk) Once those two variables were included, the paper said, humans no longer looked exceptional relative to phylogenetic expectations. Oxford summarized that result as evidence that upright walking and larger brains are central to the emergence of human handedness. (journals.plos.org) ### How does walking on two legs connect to using one hand more? The Oxford release said relative arm-to-leg proportions helped capture the shift toward bipedal locomotion. In the researchers’ account, freeing the hands from locomotion created more room for manual specialization, while later brain expansion strengthened that bias. (journals.plos.org) The paper’s wording is narrower than a simple cause-and-effect claim. Püschel and co-authors wrote that bipedalism and neuroanatomical expansion were “likely key drivers” of uniquely human lateralization, not the only forces involved. That leaves room for other influences, including development, genetics and culture, which the study says still need more work. (ox.ac.uk) ### What does the paper say about extinct human ancestors? Oxford said the same models were used to estimate likely handedness in extinct hominins. The result was a gradient rather than a sudden jump: Ardipithecus and Australopithecus were inferred to have only mild rightward preferences, while the bias strengthened with the appearance of the genus Homo and grew through Homo ergaster, Homo erectus and Neanderthals before reaching its strongest form in Homo sapiens. (journals.plos.org) PLOS Biology similarly reported that strong handedness appears to have evolved early in hominin evolution, while the direction of that bias intensified later with the genus Homo. That distinction matters because the study separates having a hand preference from having the extreme rightward skew seen in modern humans. ### What questions are still open? The April 27 paper does not settle why left-handedness persists or how much human culture helped stabilize right-handedness once it emerged. (ox.ac.uk) Oxford said those remain open questions for future research, along with whether similar evolutionary pathways shaped other forms of brain and behavioral asymmetry. (journals.plos.org) The next place to track the work is the PLOS Biology paper and any follow-up comparative studies from the Oxford and Reading researchers. The published dataset already covers 41 primate species, and the authors say it provides a framework for testing broader questions about human-specific adaptations and behavioral asymmetry. (journals.plos.org) (ox.ac.uk)