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A 555-million-year-old worm had a predilection for turning right, possibly indicating the oldest known example of handedness.

Although these worms lacked limbs and so couldn’t be considered left- or right-handed in the way that we understand, the development of a tendency to favour one side over another is evidence of an advanced nervous system.

It remains a feature of free-living mobile life today, but until this discovery, it wasn’t thought to have emerged until the Cambrian Period, which began around 541 million years ago.

at the American Museum of Natural History in New York and his colleagues analysed 100 fossil specimens of a small flatworm-like creature, Spriggina floundersi, collected in South Australia over recent decades.

These animals lived during the Ediacaran Period, when multicellular life first became widespread. It preceded the Cambrian explosion, when animal life diversified dramatically and many groups of animals first appeared.

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Spriggina lived in what was, half a billion years ago, a shallow ocean and is thought to have foraged on or close to the seafloor, moving by wriggling to the left or right.

“We have around 50 specimens of Spriggina that are clearly bent,” says Evans. Twice as many of the fossilised worms are bent to the left than to the right, he says. This means the creature itself bent to the right, as the specimens are mirror-image impressions of the animals, made when storms buried them in sand.

“This appears to be statistically significant and matches what biologists find when they study handedness in different animals today,” says Evans. “Some specimens have multiple bends to both the right and left, suggesting that they all could bend both ways, which makes sense if you don’t want to be stuck moving in a circle.”

While the majority seem to demonstrate right-handedness, it is hard to tell if any were left-handed, he says. “I imagine it’s like taking a picture of 100 people waving with one hand today. You would likely be able to count that more people are waving with their right hand, but you wouldn’t be able to tell who is right- or left-handed.”

Discoveries like this demonstrate that many foundational characteristics that are common to a variety of animals today, such as the ability to move around, bilateral symmetry and handedness, evolved in the Ediacaran, says Evans.

In the Cambrian, organisms built on that foundation to become more complex, for example adding legs to move more efficiently, becoming “less alien and more like the major groups of animals we know today”, says Evans. “This is cool because it suggests that, while the Cambrian was an amazing time in animal evolution, those organisms didn’t just come out of nowhere: they built on the foundations established in the Ediacaran.”

“The presence of handedness in any kind of functional asymmetry, really deep into the fossil record, gives us important and interesting information about how these behaviours have evolved and how deeply in time they emerged,” says at Flinders University in Adelaide, Australia.