One of the greatest enigmas of human evolution has long puzzled scientists: Why are humans overwhelmingly right-handed?
About 90% of humans, regardless of culture, prefer to use their right hand, a level of dominance not found in any other primate species. Researchers have spent decades studying the brain, genetics and development that underlie laterality, but why people became predominantly right-handed has remained unclear.
A new study led by researchers at the University of Oxford points to two major evolutionary milestones: walking upright and the spectacular growth of the human brain.
The research, published in PLOS Biology, was carried out by Dr Thomas A. Püschel and Rachel M. Hurwitz from the School of Anthropology and Museum of Ethnography, Oxford, together with Professor Chris Venditti from the University of Reading. The team analyzed data from 2,025 monkeys and primates, representing 41 different primate species.
Using Bayesian modeling, which took into account how species are evolutionarily related, the researchers tested several major theories regarding the origins of laterality. They examined factors such as tool use, diet, habitat, body size, social structure, brain size and movement patterns.
Bipedal walking and brain development
At first, humans were distinct from all other primates included in the analysis. However, the situation changed once the researchers introduced two key traits into their models: brain size and the ratio of arm length to leg length, which is commonly used as an indicator of bipedal walking.
Once these characteristics were taken into account, humans no longer seemed to be such a striking evolutionary exception. The findings suggest that the combination of bipedal walking and larger brains could explain why humans have developed such a strong right-handed preference.
The study also allowed the researchers to estimate the likelihood of laterality in extinct human ancestors. Their results suggest that early hominids such as Ardipithecus and Australopithecus probably exhibited only a slight right-handed preference, similar to that seen in great apes today.
This pattern seems to be significantly strengthened with the appearance of the genus Homo. Species such as Homo ergaster, Homo erectus, and Neanderthals likely had increasingly strong right-handed preferences, which eventually led to the extreme dominance seen in modern humans.
The Curious Case of the “Hobbit” Species
One species broke away from this trend: Homo floresiensis, the diminutive species often nicknamed the “hobbit” because of its diminutive size. The researchers estimated that this species had a much weaker right hand preference.
According to the team, this finding fits into the broader evolutionary pattern. Homo floresiensis had a relatively small brain and retained physical adaptations for both climbing and walking, rather than being completely specialized for bipedal locomotion.
The researchers believe that the evidence points to a two-stage evolutionary process. First, upright walking freed the hands from the strain of locomotion, creating new pressures that favored a more specialized and asymmetrical use of the hands. Later, as the human brain became larger and more complex, the preference for the right hand became much stronger and more widespread.
Why are there still lefties?
The study also raises new questions for future research. Scientists still don’t fully understand why left-handedness has persisted throughout human evolution, or how human culture may have helped reinforce right-handedness over time.
The researchers are also interested in whether similar limb-use preferences seen in animals such as parrots and kangaroos might point to deeper evolutionary patterns shared by very different species.
