Creating abstract patterns was a crucial development in human behavior. The earliest tools weren't expressive — just smash that bone with this stone hammer. Symbols, even basic patterns, were different.

"What they could do with symbols is, for the first time, store information outside of the human brain. And that is a major advance,” said Christopher Henshilwood, an archaeologist at Norway's University of Bergen and an author of a report published Wednesday in the journal Nature. Crucially, other people could read this out-of-brain info. Put another way: We've been using hashtags to send notifications for tens of thousands of years.

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Henshilwood's colleague, University of the Witwatersrand archaeologist Luca Pollarolo, was cleaning fragments of rock from Blombos Cave when he found perpendicular lines scrawled on a flake of silcrete rock. The study authors determined that the medium was ocher, an iron-rich rock that can be as soft as lipstick.

Ocher was important to the ancient Blombos inhabitants. They carried ocher to the cave from sites nearly 20 miles away. Henshilwood, who has been studying Blombos Cave for decades, previously discovered an ocher workshop in the cave. Using seashells as paint cans, someone swirled ocher, charcoal and seal fat into paint 100,000 years ago. (Those abalone shells were also the first evidence of a container, Henshilwood said.) Dried ocher paint still clung to the inside of the shells. A stick used to mix the paint was nearby.

It's not exactly clear what purpose the ocher served. Perhaps, Henshilwood speculates, early humans ate it as an antidote for spoiled food or slathered it on their skin as a proto-sunscreen. And at some point between 100,000 and 73,000 years ago, they figured out how to use ocher strips like crayons, the archaeologists said.

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After Pollarolo identified the hashtag, the authors of the new study attempted to re-create the pattern. They used stone flakes and pieces of ocher from Blombos to draw on rock. “There was absolutely no doubt that these were drawn with an ocher pencil or an ocher crayon,” Henshilwood said. “We could even tell the direction that that ocher pencil was drawn across the surface.” The crayon made stroke marks, the way a paintbrush does across a wall. The hashtag piece is a fragment of a larger drawing, the authors determined.

The scientists “used a battery of impressive techniques to demonstrate crayon stroke direction, the method of ocher application” as well as the ocher's chemical composition, said archaeologist Lyn Wadley, from the University of the Witwatersrand in South Africa, who was not involved with this research. It is “perfectly feasible,” she added, that the Blombos inhabitants could make pattern drawings.

"This would indeed be the oldest set of such lines that is made with an ocher 'crayon' rather than a sharp instrument, and constitute the oldest evidence of drawing with a crayon,” said George Washington University paleoanthropologist Alison Brooks.

But not all scientists were convinced that because this pattern exists it must be meaningful. The study “fails to demonstrate that the cross-hatched patterns were intentionally made by humans as a sign of symbolic representation,” said Maxime Aubert, an archaeologist at Griffith University in Australia who was not involved with this report. “We can’t exclude the possibility, for example, that this could be the result of sharpening the tips of ocher pieces on stone."

Henshilwood and his colleagues did not attempt to discern what this pattern could mean. But they point out that the crosshatch markings were important to early humans. At the very least, our ancestors made hashtags over and over. A lot.

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Archaeologists have found shell beads and ocher pieces at Blombos Cave carved with similar crosshatches. They've found it carved elsewhere in southern Africa, too. “It's found in Australasia, it's found in Asia, it's found in Europe,” Henshilwood said.

Brooks said that the similarity of these ocher lines to the engraved ones makes the case for intentional drawing. But, she said, “it is hard for us moderns to read the context.” Perhaps adults made them. Maybe they represent a signature or tally.

Or, she said, maybe they're the scribbles of children. “The very existence of such marks, however, expands the known repertoire of expressive capabilities among early members of our species in Africa,” she said.