Here’s what you’ll learn when you read this story:

• Until now, the reason why most humans tend to be right-handed and have such a strong hand preference to begin with was unknown.
• Researchers who compared traits throughout many species of primates found that humans stand out because of bipedalism, along with superior brain size and complexity.
• It’s now thought that we have such a strong preference for right-handedness because Homo sapiens adapted to walking on two feet, then evolved larger brains.

Left-handers make up roughly 10% of every human population on Earth. That ratio is so consistent across cultures and centuries that it practically begs for an evolutionary explanation. We know handedness is something we’re born with, but scientists have long puzzled over what drove our species to favor one side so overwhelmingly.

The human tendency to prefer the right hand is quite different from other primates, who tend to be right- or left-handed in equal proportions. Unsurprisingly, many previous theories have sought to explain human right-handedness through the traits that supposedly make us different from our relatives. For example, human language use is controlled by the left brain hemisphere, as is the right hand, so one theory posits that gesturing while talking led to our right-hand bias. Similarly, our use of tools, our fondness for fighting, and our cultural practices have been marshaled as explanations.

But now, new research by anthropologists Thomas A. Püschel and Rachel M. Hurwitz of Oxford University, and evolutionary biologist Chris Venditti of the University of Reading, demonstrates that none of those theories is satisfactory—and offers a better explanation backed up by statistics. The researchers examined our preference for right-handedness by sifting through pages and pages of research covering everything from genetics to human development. After testing theory after theory, they found that none could explain our right-handed exceptionalism, so they reached for a new approach.

“No combined phylogenetic comparative and meta-analytical method has been used to study anthropoid handedness, leaving key eco-evolutionary hypotheses untested,” they wrote in their study, which was recently published in PLOS Biology. “This approach offers several advantages, including better identification of variation drivers, improved control of data biases, and more accurate phylogenetic conclusions.”

Human handedness does have something of a genetic and neurological background, and some uncertain developmental research has claimed that hand preference develops in utero, but it’s most often been studied through observing other primates in the wild. That method can be problematic. Just because a chimp or gorilla reaches for something with one hand doesn’t necessarily mean that hand is dominant, but it’s often misconstrued that way. Püschel, Hurwitz, and Venditti instead decided to get a more accurate idea of handedness in primates by focusing on hand-use direction and strength among species.

The team modeled the evolutionary relationships between 41 monkey and ape species. They also set out to test existing hypotheses about the evolution of handedness as it relates to factors such as habitat, brain size, body mass, locomotion, diet, tool use, competition for mates, and socialization. They found that no hypothesis predicted hand preference more accurately than another. Humans (unsurprisingly) had the highest occurrence of hand preference, with an overwhelming bias towards the right hand. Gorillas, chimps and red-tailed monkeys showed a weak to moderate preference for their right hand, still nowhere near humans, while orangutans and golden snub-nosed monkeys were biased toward their left.

Something that did surprise the researchers was a potential link between human handedness and bipedalism. As human ancestors transitioned from climbing trees to walking on terra firma, arms evolved to be shorter while legs grew longer. Humans also have the largest and most complex brains of all living primates. As each ancient hominin species evolved to be more advanced than the next, brain size eventually tripled. From around 800,000 to 200,000 years ago, drastic climate changes pushed rapid brain evolution. Facing an unpredictable environment, hominins and early humans adapted by processing more information, which gave them an advantage in interacting with each other and their surroundings.

Homo sapiens are an evolutionary anomaly because of brain size and bipedal locomotion. But when the research team compared our species to other primates using the other traits that could possibly explain handedness, like habitat, diet, and social systems, humans were no longer outliers. So our overwhelming bias towards the right hand must have evolved because of bipedalism and superior brains. Analysis of Australopithecus, Ardipithecus, and other human ancestors showed that they most likely had just a slight preference for the right hand, making them more like great apes than modern humans. The right-hand bias would grow stronger when Homo species such as Homo erectus and Neanderthals appeared.

“These results imply that our unusual gait was the main initial driver of our exceptional handedness strength with our large brain more linked to the directionality,” the researchers said. “In humans, the evolution of bipedalism and the subsequent freeing of the hands may have intensified selective pressures for stronger hand preferences.”

The evolutionary explanation turns out to be as elegant as the mystery: walking upright freed our hands, and a rapidly expanding brain cemented a directional preference. As for the 10% who reach the other way, they remain one of the most intriguing unanswered questions in this human evolution story.