Around 90% of people—across cultures and throughout recorded history—are right-handed. That fact may seem ordinary, yet it remains one of the deepest puzzles in human evolution: how does a single trait become so overwhelmingly dominant in a species? A new study may offer some answers by tracing humanity’s right-handed bias to our early ancestors.
A team led by researchers from the University of Oxford suggests that humans became overwhelmingly right-handed because of two major evolutionary milestones: walking upright and developing bigger brains. In a new study, published in PLOS Biology, researchers tested previous theories on the origin of one of humanity’s most distinctive traits to offer a new hypothesis.
Raise your right hand
Humanity’s right-hand bias differentiates us from other primates, with their preferences being more evenly split across the population. Scientists believe that hand preference begins before birth, and yet it isn’t determined by a single right-handed gene. So, how have humans developed a population-level preference on this scale?
To help find an answer, the researchers behind the new study examined data on 2,025 individuals across 41 species of monkeys and apes. They then tested major theories on handedness based on a few key factors, including the use of tools, diet, habitat, brain size, and movement patterns.
Those initial traits did not explain why humans prefer their right hand. However, when the researchers factored in a large brain and the ratio between the length of arms and legs, humanity’s overwhelming right-handedness no longer stood out as an anomaly. That means those two traits are best at estimating the handedness of earlier human ancestors, the researchers argue.
“Our results suggest it is probably tied to some of the key features that make us human, especially walking upright and the evolution of larger brains,” Thomas Püschel, associate professor in evolutionary anthropology at the University of Oxford, and lead author of the study, said in a statement. “By looking across many primate species, we can begin to understand which aspects of handedness are ancient and shared, and which are uniquely human.”
Evolutionary bias
Early human ancestors began walking on two legs, “freeing the hands from the work of locomotion” and creating new evolutionary pressure for specialized, lateralized hand use, according to the press release. Then came bigger and more reorganized brains, as seen in species such as Homo erectus and eventually Homo sapiens, causing that right-handed bias to harden into the near-universal pattern seen today.
The recent study showed that early hominins such as Ardipithecus and Australopithecus may have had a slight preference toward using their right hand, similar to the pattern seen in great apes today. The right-handed preference became much more prominent with the genus Homo, the group of species that includes modern humans and our closest extinct relatives. Homo ergaster, Homo erectus, and Neanderthals all showed a strong right-hand preference, which reached its modern extreme in Homo sapiens, according to the research.
There was one exception: Homo floresiensis. The small-bodied humans discovered in Indonesia showed a much weaker right-hand bias, likely because they had relatively small brains and maintained a mix of walking upright and climbing rather than adapting full bipedalism.
The researchers believe there are still unanswered questions behind humanity’s overwhelming right-handedness, including whether culture plays a role in maintaining one hand’s preferred use over the other and why left-handedness has persisted at all.
