As anyone who grew up favoring their left-handedness can attest, the world overwhelmingly caters to right-handers. The vast majority of humans (about 85-90 percent) are right-handed, a tendency influenced by a combination of genes and environment. But it turns out that this well-known rightward bias may go back much further than scientists once thought:hundreds of millions in fact, many years ago. This rightward preference may have been shared by some of the oldest known multicellular organisms.

That is the central argument presented in a study published this week in the journal Scientific Reports. The international team of researchers examined more than 100 well-preserved fossils of Sprigina flounderan ancient, inch-long worm-like animal that made its way across southern Australia about 550 million years ago. Most of those fossils had something in common: they all showed the creature favoring its left side. And since fossils are mirror impressions captured at one moment in time, that actually means that animals preferred their right side throughout life.

The researchers say this represents the oldest known example of this type of asymmetry in nature, and may lend credence to the idea that laterality is deeply rooted in the evolutionary history of complex living creatures.

“The predominance of curvatures to the left in the fossils of sprigina “suggests a preference for right turns in life and represents the oldest evidence of lateral behavior among animals,” the researchers write in the paper.

Evidence of ancient laterality, trapped in stone.

sprigina The fossils date back to the Ediacaran Period, an era of immense biological change and adaptation. This period was responsible for introducing the first multicellular animals visible to the naked eye. Although they may not seem like much today, sprigina and their contemporaries were some of the first animals capable of movement and other abilities scientifically considered complex behaviors. These are among the first organisms that many people would identify as real animals, rather than just blobs of goo.

He sprigina The study’s fossil analyzes came primarily from the Nilpena Ediacara National Park, located in the western part of the Flinders Ranges in southern Australia. It is a remote area that is home to some of the best preserved fossils from the Ediacaran period. In this case, Spriggina’s fossil collection was buried together during some storm about 550 million years ago.

And although it may not seem like it, the ancient hammerhead worm has some key things in common with humans. Specifically, it is among the earliest known examples of an animal with bilateral symmetry, meaning it has a distinguishable front and back, left and right, and top and bottom. This is also true for humans and most animals today, but it wasn’t always like this. The findings suggest that favoring one side or the other is not a recent adaptation developed by humans, but is possibly something deeply ingrained in the way complex animals have evolved. While interesting in its own right, the research falls short of answering why this is the case.

“When we talk about being right- or left-handed, most people probably think about how they hold a pencil or kick a soccer ball,” American Museum of Natural History assistant curator of invertebrate paleontology and lead author of the study Scott Evans said in a statement. “But our research shows that an animal without hands or feet, which lived more than 500 million years ago, may have had its own version of handedness.”

Humans and laterality: it’s in the genes

The fact that the vast majority of human beings today are right-handed is not entirely explained by genetics. Certain cultures in Asia and Africa have traditionally considered being left-handed to be impure. Others have gone even further, baselessly suggesting that it was a sign of the influence of satanic forces. Those and other cultural forces have led to generations of people who naturally prefer their left hand to being forcibly removed from it. The tools and basic infrastructure are also largely designed around right-handedness, which can further pressure those who are naturally inclined to favor their left to conform to the social standard.

Related: [Why are most people right-handed?]

But still, even taking those factors into account, laterality has a strong genetic component. Studies show that up to 40 genes may play some role in determining handedness, which can occur even before birth, in fetuses. Although it’s still not entirely certain what causes some people to prefer the left hand, some researchers attribute it to random mutations during early brain development.

And while lefties have typically had to work harder in many ways, their minority status offers some unique advantages, especially in combat and sports. In fact, some modern athletes today, such as tennis legend Rafael Nadal, were born favoring their right hand, but were trained to use their left specifically to gain the slight advantage it offers.

Obviously, quite a lot happened in the 548 million years between sprigina and early human ancestors, which may explain the dominance of the right hand today. Still, the findings suggest that this ancient biological bias may have a much older origin story than anyone expected.

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