About half a million years ago, the ancestors of modern-day humans did not just eat freshwater shellfish. They also created a form of art using mollusk shells.

Josephine Joordens from the Leiden University in The Netherlands, together with colleagues, found evidence that our primitive relatives used shells as a canvas of art in an ancient shellfish. The said shellfish has been housed in the Naturalis Biodiversity Center in The Netherland for more than a century.

The shell was excavated along with hundreds of others from a riverbank in Java, Indonesia in the late 19th century by Eugene Dubois, whose interest in Charles Darwin's Theory of Evolution led to his discovery of the Java man, now known as the Homo erectus, in 1891. The Homo erectus species thrived between 1.9 million and 100,000 years ago and is believed to be the ancestor of modern-day humans (Homo sapiens).

Joordens first noticed the shell's mysterious geometric engravings, which have slashes and an M-shaped zigzag, seven years ago and confirmed after several years of study that they were not caused by natural weathering processes or made by an animal. Instead, they were likely made by a human and appear to have been etched by a sharp object such as a shark tooth.

By using two dating methods, Joordens and colleagues found that the shell is between 430,000 and 540,000 years old; the ancient artwork could possibly be the oldest geometric carving to be found as it is four times older than the earliest known engravings found in South Africa.

Although the researchers do not have a clear idea about what the engraving on the ancient shell means, the artwork shows that our human ancestors are smarter than previously believed.

"The manufacture of geometric engravings is generally interpreted as indicative of modern cognition and behavior," the researchers reported in their study published in the journal Nature on Dec. 3. "This discovery suggests that engraving abstract patterns was in the realm of Asian Homo erectus cognition and neuromotor control."

By attempting to engrave on freshwater shells, the researchers found that making the carving was not an easy task.

"You had to use a lot of strength in your hands," Joordens said. "You had to be precise to make those angles. [But] if you engrave that dark surface and the white appears, that must have been quite striking for Homo erectus."