When piecing together the story of human capabilities, one of the most useful sources of evidence available is the presence or absence of an ability in other species. Humans make art; chimpanzees do not. This gives us some clues about the time bracket where we should search for the emergence of symbolic and abstract thinking.

It wasn’t clear whether extinct species of humans like Neanderthals engaged in these behaviors until earlier this year, when a group of researchers announced evidence of Neanderthal etchings in a cave wall from more than 39,000 years ago. Now, a new paper in Nature reports a more startling discovery: etchings on a shell that date back to 500,000 years ago, created by an entirely different species: Homo erectus. The shell was actually found with the first Homo erectus skeleton, Java Man, but has sat in a collection until recently re-analyzed.

The intentional creation of abstract patterns is seen as a major step in cognitive evolution, no matter how simple the patterns. It is “generally interpreted as indicative of modern cognition and behavior,” write the researchers who discovered the shell etchings. If Homo erectus was carving abstract patterns, it means that they were capable of more advanced cognition and motor control than previously thought.

The first evidence of Homo erectus was discovered by Eugène Dubois in 1891, at Trinil in Java, Indonesia. The species lived between 1.9 million and 140,000 years ago and migrated from its origins in Africa into Asia. It is thought by some to be a direct ancestor of both modern humans and Neanderthals.

When Dubois returned home to the Netherlands after his excavations in Java, he brought with him the fossilized bones that were evidence of Homo erectus, but also other objects found with the bones, including shells. It was while examining these shells that researchers recently noticed a faint pattern engraved on one. The pattern itself is simple: two small zigzags, and some parallel lines, made with a shark tooth. It isn’t possible to tell whether these scratches are an artistic expression, an idle doodle, or perhaps a mark to indicate the shell as an individual’s possession.

What is certain, however, is that the marks are intentional. The researchers tried to recreate the pattern and found that considerable force and control was needed to create scratches that deep. The lines of the zigzags meet precisely, with no gaps, the researchers write, “suggesting that attention was paid to make a consistent pattern.” Although the fossilized shells are white, a fresh shell would have been brown, and a scratch on the surface would have created a white line that would have shown up in striking contrast with the dark surface.

The researchers examined the weathering of the grooves to establish that the scratches were made before the shells were buried in sediment along with the fossilized Homo erectus bones. They also analyzed the sediment found inside the shells and found that it was approximately 500,000 years old. This is a more recent date than previous estimations for fossils from this site, but it's likely to be more accurate.

It’s clear that the shells in the Dubois collection were not a natural, living population of shellfish that was buried along with the Homo erectus bones, write the researchers. For one thing, a natural population would contain shells of a range of different sizes, whereas all the shells in the collection were large, adult specimens. This wasn’t due to the archaeologists picking only the biggest samples, because the collection contains fossils of all sizes. The shells also come from different sources that would not have landed in the same place without being collected artificially.

But most importantly, the shells all had characteristic holes at the particular place needed to damage the muscle of the mollusk and open the shell easily. Based on attempts to recreate these holes with various tools, the researchers suggest that they were made deliberately, again using shark teeth. This means that the shells were gathered by individuals who chose the biggest specimens and used tools to open them.

Tool use on its own is not unexpected—there is already evidence of Homo erectus using tools, and other living primates have been observed using tools, including capuchin monkeys. However, the sharpening of one of the shells, like a knife, is the earliest known use of shells as raw materials for tools, and it helps to explain the lack of stone tools from this period in Java.

The discovery of the engraving was highly unexpected, the researchers note, because it is at least 300,000 years older than the next oldest indisputably intentional engraving. However, they add, it is only a matter of time before more evidence is discovered that can fill the gap between this very early case of abstract behavior and the more recent cases that are well established.

Nature, 2014. DOI: 10.1038/nature13962 (About DOIs).