Scientists have stumbled upon the oldest known shell to have been engraved by an early human - a Homo erectus from about half a million years ago. They found it in a Dutch museum collection, where it sat untouched since the 1930s.

Researchers believe a Homo erectus etched a zig-zag pattern on top of the freshwater mollusk shell with a shark tooth or another sharp object some 430,000 to 540,000 years ago. The engraved shell is at least 300,000 years older than any other undisputed engravings of this nature.

"It's something that we don't expect to find in deposits of that age," says Josephine Joordens, a post-doctoral researcher at Leiden University in the Netherlands and one of the researchers on the paper published in Nature.

The shell came from a site in Java, Indonesia, where scientists have previously found remains of Homo erectus. Scientists have long debated whether geometric engravings are solely a human behaviour.

"It's a species that before we have thought wasn't capable of making things like this," she says. "... Something that we recognize as typically human was produced by Homo erectus."

'Almost impossible' to determine what it means

It's "almost impossible" to know what — if anything — the zig-zags represent. But, Joordens believes it was created with intent because of the difficulty of the task.

The person who made it must have put effort in it and must have had some kind of intention or idea to make it. - Josephine Joordens,

"You really have to be very careful. You have to have strong hands," says Joordens, adding that scientists attempted to create a similar engraving on a fresh shell. "... The person who made it must have put effort in it and must have had some kind of intention or idea to make it. But what that was, we simply cannot know."

Joordens says it's "really remarkable" that the shell survived and didn't separate or disintegrate. After it was discarded, Joordens believes the shell was quickly covered by sediments, which helped preserve its condition.

It lost some of its outer layer over time, but a faint remnant of the engraving remains because the Homo erectus etched it deep into the shell.

Discovered 'purely by coincidence'

The discovery of the engraved shell started "purely by coincidence," Joordens recalls.

Joordens knew the Naturalis Museum's Dubois collection contained "box after box after box" of fossil freshwater mussel shells — 166 to be exact — that had last been studied in the 1930s.

The fossilized Pseudodon shell with the engraving made by Homo erectus at the Trinil site in Indonesia. (Wim Lustenhouwer/VU University Amsterdam) When Stephen Munro, a colleague who studied shells, made a stopover in Amsterdam en route to Ethiopia, Joordens suggested he go through the museum's shell collection. Short on time, Munro photographed the specimens to study later.

"It was only when he looked at the photos that he saw it for the first time," she says.

It takes the perfect storm of conditions to notice the engraving if you're not expecting it, Joordens explains, saying it has to be held under the right light at a certain angle to stand out.

"So many people must have had the shell in their hand, but they simply missed it," she says.

Munro sent Joordens and a group of other scientists the photo to see if they could determine what the markings were.

"Then, little by little, you are beginning to realize that it may be something — something real and something important," she says.

The shell will be on display at the museum starting Wednesday.

'A real shell culture' for Java's Homo erectus

The researchers also found one shell from the collection that had a smooth, polished edge and may have been used as a cutting or scraping tool.

Wil Roebroeks, left, Josephine Joordens, centre, and Frank Wesselingh, right, look at the Naturalis museums new special exhibit featuring the shell they helped discover. (Marc de Haan/Leiden University) ​Homo erectus in Java appeared to have "a real shell culture," says Joordens. They ate fresh water shellfish and used the shells to produce tools and as a canvas for some of the earliest known drawings.

"All that together is kind of exciting," she says.

The scientists hope to continue their research and plan to re-examine the shells and spend some time looking at other understudied aspects of the Dubois collection.

"We know that once you start looking and you start looking very well, you find things that you don’t expect," she says.