Scientists say that a series of marks carved onto a freshwater clam shell represent the earliest known engravings made by a human ancestor.
The zigzag lines visible in the photo above probably were carved at least 430,000 years ago by a Homo erectus, an extinct hominid with a brain only slightly smaller than those of modern humans, researchers say.
The findings were published Thursday in Nature.
The previous earliest known engravings also have a zigzag pattern and were found in the small Blombos Cave in South Africa, about 180 miles east of Cape Town. They date back about 100,000 years.
"I was pretty surprised when I first heard about this because it was not what I would have predicted," said Pat Shipman, an adjunct professor of anthropology at Penn State, who was not involved in the study. "But having gone through the evidence carefully, I find it quite convincing."
This earliest known engraving has been hiding in plain sight for more than 100 years.
The fossilized shell was collected in Java in 1891 by Dutch physician and anthropologist Eugene Dubois. Dubois traveled to the Indonesian island in search of fossils that would link man and ape on the evolutionary tree. When he returned, he brought back fossils of Homo erectus, as well as a wide range of other fossils, including elephants, fish bones and a few hundred shells. The collection is housed in the Naturalis Biodiversity Center in the city of Leiden.
Dubois described a species of clam from the shells he found, and at least one other researcher did extensive work on the shells in the 1930s, but the marks on the surface of the shell defied detection until May 2007 when an Australian doctoral student started photographing the shells so he could study them at home.
"He couldn't stay in the collection, so he was just photographing all of it," said Jose Joordens, researcher at the Faculty of Archeology at Leiden University and the lead author on the paper. "As soon he photographed it, he saw it right away."
To figure out what these marks were, and how they got there, Joordens and her colleagues began by taking a closer look at all the shells in the collection. They noted that one-third of them had one or two small holes in the same area. This led the researchers to surmise that the shells had been collected by an animal that got to the meat in the shell by poking the hole.
Next they asked what animals living at this time could have made the hole. After ruling out all the options -- including otters, rats, birds and octopus -- they concluded that only Homo erectus could have made this hole, probably using a shark tooth as a tool.
Trying this ancient technique for themselves, the researchers found that if they poked a clam shell with a shark tooth at just the right spot, they could cut through the muscle that holds the shell shut.
"If you pierce the shell and damage the muscle, it is easy to open the shell, and you can eat them," said Joordens.
Now the scientists had evidence that the shells had been deliberately assembled, and that they had been assembled by Homo erectus. Analysis of the grooves on the shell show they were made with a hard tool, and that they were made before the shell fossilized.
"To me, the most logical assumption is they made the carvings in the context of opening the shells for food," said Joordens. "They had the shark tooth in one hand, and the shell in another. It is such a small step to use the the shark tooth to play around a bit on the shell."
Joordens imagines the Homo erectus scraping the tooth along the brown surface of the shell, revealing a layer of white.
"Maybe the person thought it was attractive, and was motivated to make another line, and before you know it, there was a drawing," she said. "I think the person did his or her best to make something that looks good."
The conundrum here is that only one of the 248 pieces of shell that Joorden and her colleagues analyzed had any evidence of engraving on it.
"Clearly everbody would like to see more," said Shipman, "but you've got what you've got."
Joorden said the next step for her team is to examine the Dubois collection even more closely for similar markings, as well as return to the original site in Java, which has remained untouched for more than 100 years.
"It's hard to look for what you don't expect, but now, we have this new way of looking," she said. "Opening up the excavation wuld allow us to check so many things, and strengthen our story."