In 2007, Stephen Munro got the shock of his life. The archaeology graduate student was studying mollusk shells gathered more than 100 years ago on the Indonesian island of Java, where an early human ancestor, Homo erectus, had roamed at least 1 million years ago. As he studied photographs of the shells, Munro spotted one apparently engraved with a pattern of zigzag lines. “I almost fell off my chair,” he says. That’s because the oldest known engravings date back 100,000 years and were made by modern humans—the only species thought to be capable of making abstract designs.

Now, after 7 years of work on the shells, Munro and colleagues have confirmed their observations. They also report that one of the shells was used as a tool of some sort, a finding that would expand the known toolmaking capabilities of H. erectus, which was thought to have made only simple tools out of stone.

“If correct it certainly pushes back in time the evidence for marking objects in a way that arguably could be considered evidence for symbolic activity,” says Curtis Marean, an archaeologist at Arizona State University, Tempe, who was not involved in the study. But he points out that the Java site, known as Trinil, was excavated in the 1890s using “quite primitive” archaeological methods and that no one has reexamined the location using modern techniques. That means, he says, that “the observation is essentially devoid of context.”

Mindful of this kind of criticism, archaeologist and team leader Josephine Joordens of Leiden University in the Netherlands says her group took its time answering a series of questions about how the shell came to be engraved as well as when it might have happened. First, the researchers looked at how the shells accumulated at Trinil in the first place. Munro had focused his research on about 166 specimens of the freshwater mollusk Pseudodon, collected by Dutch paleoanthropologist Eugène Dubois at the site where he found now-famous H. erectus fossils. Studying cigar boxes full of mollusks from the site that are now housed in Leiden, Joordens and other team members found that a third of the shells had holes right where a muscle that keeps the shell closed is found. These holes were apparently made by humans using shark teeth, also found at the site, as tools to open the shells so they could eat them; when team members did their own experiments trying to open shells with sharks’ teeth, they got a very similar pattern of holes.

Once they had established that humans gathered the shells, the researchers set about determining whether the engraved shell had been etched deliberately or could have been scratched by rocks in the water or by animal teeth. For those studies, the team brought in Francesco d’Errico, an archaeologist at the University of Bordeaux in France known for his studies of similarly engraved objects in Africa and Europe. Using a microscope, d’Errico was able to demonstrate that the marks had been etched in one session, by one person using a sharp tool; especially revealing were the “turning points” at the ends of the zigzag pattern, which indicated that the engraver had kept the tool firmly on the shell as he or she reversed direction.

But could the pattern be a hoax? Robin Dennell, an archaeologist at the University of Exeter in the United Kingdom, says he discussed that possibility with the team. “I raised the scenario that one of Dubois’s workforce might have been bored and engraved a shell over lunch time.” But the detailed studies of the engraving, the team reports online today in Nature, revealed that the interiors of the grooves were smooth and rounded, compared with the “jagged and sharp-edged” grooves that team members made themselves on ancient Pseudodon shells. That’s a telltale sign that weathering of the engraving had taken place after the shells were buried in sediments at the Trinil site. Dennell is persuaded. “That part of the analysis is water-tight,” he says.

Yet even if ancient humans engraved the shell, says Russell Ciochon, a paleoanthropologist at the University of Iowa in Iowa City, the team has not shown that H. erectus did it. Ciochon, who has spent many years working at sites in Java, agrees with criticisms that the shells have been taken out of context, because Trinil was not an occupation site where early humans actually lived. Rather, Ciochon argues, the human fossils found there (which include a skullcap widely agreed to be H. erectus and a thigh bone that could belong to either H. erectus or H. sapiens, a matter of sharp debate) were washed into the site by a powerful flood, and nothing found with them—including the shells—can be assumed to have been associated with them originally. Although the team dated four of the shells in the collection, including the engraved shell, to about 500,000 years ago using two different techniques on sediments of sand and clay found inside them, Ciochon says that those sediments could have entered the shells during the earlier flood event that created the site, and that H. sapiens still could have come along much later and performed the etching.

Then there’s the question of whether the pattern on the shell is truly evidence of rudimentary symbolic behavior. Iain Davidson, an archaeologist at the University of New England in Armidale, Australia, cautions against attaching any meaning to the etchings. According to evolutionary theory, Davidson argues, humans must have been making markings for nonsymbolic reasons—simple doodling, perhaps—before natural selection gave them the cognitive ability to turn those markings into abstract symbols.

So far, of course, there is only one example of such a potentially ancient engraving to study. But now that it has been found, Joordens says, she hopes that other researchers will search their own collections for other samples. “One finds what one expects, but we never expected to find this.”