Oxford-led PLOS Biology paper argues bipedalism freed hands from locomotion and brain expansion hardened a rightward bias into near-universal human handedness.
Key Takeaways
Study: adding brain size and arm-to-leg ratio (bipedalism proxy) to primate models makes human handedness statistically unremarkable, not an anomaly.
Predicted handedness gradient across hominins: mild preference in Ardipithecus/Australopithecus, strengthening through Homo erectus, peaking in Homo sapiens.
Homo floresiensis (small brain, mixed locomotion) is the key exception: model predicts weak preference, consistent with the two-factor framework.
Two-stage mechanism proposed: upright walking created selection pressure for lateralized manual skill; brain expansion then locked in the rightward direction.
Open questions remain: why left-handedness persists, the role of cumulative culture, and whether convergent lateralization in parrots and kangaroos reflects a deeper pattern.
Hacker News Comment Review
Commenters flagged that most coverage conflates two separate traits: strength of lateralization (how committed) vs. direction (which hand), which the paper treats as independently evolved by millions of years.
Several commenters noted the paper does not clearly explain why right rather than left won out, only why strong handedness emerged at all, leaving the headline question partly unanswered.
Anecdotal clustering of left-handedness in families and cross-sport mixed dominance raised interest in developmental timing and environmental correction as confounds the study does not address.
Notable Comments
@jnakano89: “Handedness is two traits, not one” – bipedalism explains lateralization strength, brain size explains direction; Australopithecus was already strongly lateralized before rightward consensus.
