It would make more sense if only a few related cultures exhibited it, but the trait is everywhere. No matter where you are in the world, the humans living there are about 90 percent right-handed while the remaining 10 percent are predominantly left-handed. This curious facet isn’t seen in our primate relatives, either.
Evolutionary biologists and neuroscientists have spent decades trying to understand why the vast majority of Homo sapiens prefer using their right limb, but have since come up…well, empty handed. According to researchers at the University of Oxford in the U.K., the answer may finally be within our grasp. After comparing behavioral, neurological, and social characteristics from 41 species of monkeys and apes with humans, they say the answer isn’t found in our hands at all. It’s in our legs.
Their findings are detailed in a study recently published in the journal PLOS Biology. Using a statistical modeling framework focused on interspecies evolutionary relationships, researchers first considered some of the most prominent theories on handedness. These included aspects like diet, habitat, body mass, social structures, tool usage, and locomotion. In every case, we humans remained outliers in patterns that otherwise might explain the attribute in other primates.
They then introduced two hypothetical influences into their comparisons: brain size and the length ratios between legs and arms. That arm-leg ratio may seem arbitrary, but it’s considered a standard reference point for bipedal movement. Once these traits were included, humanity’s handedness exception disappeared entirely. Basically, big brains and long legs correlate directly with dominant hands.
“This is the first study to test several of the major hypotheses for human handedness in a single framework. 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,” study co-author and University of Oxford evolutionary anthropologist Thomas Püsche 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.”
The new approach meant that Püsche’s team didn’t have to stop there. With the same modeling, researchers estimated handedness preferences across extinct human ancestors. The results align with a slow evolutionary shift towards the right limb. Early hominin species like Ardipithecus and Australopithecus likely only had slight leanings towards right-hand dominance comparable to present-day great apes. However, the arrival of the Homo genus saw increasing right-handedness through Homo ergaster, Homo erectus and Neanderthals. The culmination can now be seen in Homo sapiens.
The study’s authors did note an interesting exception to the rule in Homo floresiensis, the famous “hobbit” ancestors native to Indonesia. At the same time, their physiology likely explains the outlier. H. floresiensis featured a small body and brain that specialized in upright climbing and walking, not full bipedalism.
With these conclusions, researchers now believe two phases took place for humanity’s transition to overwhelming right-handedness. Ancient ape ancestors first started walking upright, which then allowed them to use their upper limbs more frequently for other tasks. As brains continued to develop and grow, rightward focus solidified in today’s H. sapiens.
“Our findings identify bipedalism and neuroanatomical expansion as likely key drivers of uniquely human lateralization, while also revealing broader ecological patterns shaping handedness across primates,” the study’s authors wrote.
From here, researchers hope to study how human cultures further entrenched right-handed dominance, why left-handed alternatives still exist at all, and if similar limb trends are visible in other animals.
“This work provides a framework for disentangling human-specific adaptations from general primate trends in the evolution of behavioral asymmetries,” the team added.
