Bipedalism, Bigger Brains Drove Right-Handedness
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- Dr. Thomas A. Püschel, Rachel M. Hurwitz, and Professor Chris Venditti published a PLOS Biology study analyzing 2,025 individual monkeys and apes across 41 species, testing multiple longstanding hypotheses about handedness origins in a single Bayesian framework.
- Once brain size and arm-to-leg length ratio (a bipedalism marker) were added to the models, humans no longer appeared as an evolutionary outlier for handedness — the two traits together accounted for the extreme right-hand skew.
- Model estimates suggest Ardipithecus and Australopithecus had only mild right-hand preferences comparable to modern great apes, with the bias intensifying sharply in the genus Homo (Homo ergaster, Homo erectus, and Neanderthals).
- Homo floresiensis was predicted to have a much weaker right-hand bias than other Homo species, a finding the team says fits its small brain and retained climbing adaptations rather than fully specialized bipedalism.
- The authors propose a two-stage evolutionary process: bipedalism first freed the hands from locomotion, creating pressure for specialized hand use, and later brain growth amplified and spread the right-hand preference.
- The researchers acknowledge the framework does not yet explain why left-handedness persists in roughly 10% of humans or how culture may have further reinforced right-hand dominance.
Why it matters: This reframes right-handedness as tied to the same evolutionary milestones — bipedalism and encephalization — that define what makes humans unique, rather than a standalone trait. Researchers can now estimate handedness in extinct hominins from skeletal and cranial data, and the model retroactively flagged Homo floresiensis as a handedness outlier from its small brain alone — a built-in natural test of the theory.
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