Archaeologist Stephen Munro nearly fell off his chair when he noticed patterns of straight lines purposefully etched on a fossilized clamshell. The engravings were half a million years old, which meant they'd been made by a Homo erectus—an extinct human species that predated Homo sapiens by upwards of 300,000 years.

In addition to the engravings, Munroe and his colleagues found shells that were carefully crafted into specialized tools. Taken together, these discoveries suggest that Homo erectus was far more sophisticated than previously believed and capable of symbolic thought.

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"It is a fascinating discovery," says Colin Renfrew, an archaeologist at the University of Cambridge. "The earliest abstract decoration in the world is really big news."

The shells, which previously had been sitting in a museum, were collected more than a century ago by Dutch archaeologist Eugene Dubois on the Indonesian island of Java. Dubois had obtained the specimens from the same excavation site where, in the 1890s, he discovered the first-known remains of Homo erectus. In 2007, Leiden University archaeologist Josephine Joordens began studying the shells, looking for clues about what the environment had been like for humanity's ancestors. It was then that her colleague, Munro, noticed the etchings.

In the seven years since, a team of scientists led by Joordens have been studying the shells, confirming their age and that the lines had not been made by animals. The results of their research have been published in Nature.

As NPR reports:

"It's not that we just have this one isolated engraving, but it forms part of a much more extensive and systematic use and exploitation of these freshwater shells," says Joordens.

Many of the shells have one or two holes right at a particular spot. The holes appear to be made with a pointed object like a shark tooth, using a rotating motion. The research team did experiments showing that drilling into this spot would poke the muscle of this shellfish and make it open.

Also, they found one shell that appears to have been shaped to be a tool for cutting or scraping.

And they did extensive microscopic analyses on the apparent engraving. Joordens notes that when the shell was fresh, it would have had a black exterior; the engraver would have produced a striking pattern of white lines on a black background.

"It's very carefully done," says Andrew Whiten, a psychologist and primatologist at the University of St Andrews in the UK. "There is a wonderfully straight section and the [etch] turns in one continuous line. That's not just intentional but careful in what strikes as a very modern way. Apes aren't doing that. It would be staggering if they did."

What the etchings meant is anyone's guess. And other scientists are skeptical.

"Maybe it's not as intentional a design as they imply," says Alison Brooks, a paleoanthropologist at George Washington University. She speculates that a Homo erectus child could have picked up a tool a parent used to open a shell, and tried it out.

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Still, she says, "It raises the possibility that the development of human cognition — human culture — was a very long process. It was not a sudden development."