"This is the first study to test several of the major hypotheses for human handedness in a single framework."
— Thomas A. Püschel, University of Oxford
A Breakthrough in Understanding Why Humans Are Right-Handed
A new study published in PLOS Biology has shed light on a deeply human trait: our overwhelming preference for the right hand. While approximately 90% of humans are right-handed, most other primates show a balanced left-right hand preference. Analyzing data from over 2,000 monkeys and apes across 41 primate species, researchers used Bayesian models to trace the evolutionary origins of this asymmetry.
The study calculated a Mean Handedness Index (MHI) for humans at 0.76, indicating a strong right-hand preference. In contrast, other primate species cluster near zero, showing no significant handedness bias.
The Two-Stage Evolutionary Process
The researchers proposed a clear timeline for the development of right-handedness, linking it to two key evolutionary changes:
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First Stage: Bipedalism. As early hominins began walking upright, their hands were freed from the demands of locomotion. This physical freedom likely favored more specialized, unimanual hand use, laying the groundwork for a dominant hand.
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Second Stage: Brain Expansion. The growth of the brain, particularly in areas responsible for complex tasks like tool use and communication, is proposed to have reinforced this right-hand bias. The study notes that sequential behaviors (hierarchical action) are often linked to the brain's left hemisphere, which controls the right side of the body.
Humans Are Not Evolutionarily Exceptional
When the researchers controlled for brain size and the intermembral index (the ratio of arm length to leg length), humans no longer appeared evolutionarily exceptional in their handedness. This suggests that two specific physical traits—a larger brain and a higher leg-to-arm ratio (a marker of bipedalism)—are the primary drivers of human right-hand dominance.
What the Fossil Record Tells Us
The study also provided handedness estimates for extinct hominins, offering a glimpse into the past:
- Early species like Ardipithecus showed only a weak right-hand preference.
- Handedness strengthened significantly in Homo erectus and Neanderthals.
- Fossil evidence suggests a 90% right-hand dominance existed approximately 2.6 million years ago, well before the emergence of Homo sapiens and Neanderthals.
- The small-brained Homo floresiensis (the "hobbit" species) was predicted to have weaker handedness, possibly due to its retained climbing adaptations.
Unanswered Questions
The study acknowledged that two major questions remain:
- The persistence of left-handedness: Why does about 10% of the human population remain left-handed?
- The role of cumulative culture: How did social learning and cultural transmission reinforce right-hand dominance across generations?
The authors suggested future research could investigate limb preferences in other animals, such as parrots or kangaroos, to better understand how and why handedness evolves.