"It is really exciting work," says archaeologist Leslie Van Gelder of Walden University in Minneapolis. "We don't have to know what it means, we just have to know that to the people making it, it meant something."

Discovering a doodle

Scientists discovered the tiny pair of bones, each about the size of an adult's thumb, at an open area known as the Linjing site in Henan Province. The region once harbored a spring, which likely drew an array of animals and hungry hominins to the region, says study author Francesco D’Errico of the Université de Bordeaux in France.

Researchers have uncovered thousands of bone fragments at the site, including remains from horses, extinct wild oxen known as aurochs, and donkey relatives called onagers. Many of the bones sport cut marks made when they were still fresh, evidence of many successful hunts. A collection of stone tools found at the site also revealed surprisingly sophisticated methods of tool shaping.

In 2016, while studying the fossil menagerie, researchers spotted clues to something even more intriguing: evidence of purposeful engraving.

For the latest work, the team began careful analysis of the shape and arrangement of the grooves in the two bones, revealing that they differed from butchery slashes in several ways. For one, the marks were much more shallow, indicating they were likely cut into a semi-fossilized rib. But the carved lines even dip into the pits in the bone, pointing to their creation with the sharpened tip of a rock as opposed to the long edge of a stone tool, which was more commonly used to cut meat.

The analysis also revealed a stunning number of details about the ancient carver. The hominin was likely right-handed, based on the lines' asymmetry and direction of carving. In spots where the tool seemed to dull, the hominin made several passes of the stone tip, etching multiple lines that nearly overlap.

Perhaps most telling, microscope images revealed red residue on one fragment, and chemical analysis showed that it contains traces of iron oxide not found in the sediments on the opposite side of the bone. This suggests that the pigments were not accidental: The mystery human relative likely smeared iron-rich clay called ochre into the lines to make them stand out.

Mysterious artists

On the surface, discovering a set of straight lines may not seem like a big deal, but "it's not so much the lines themselves, it's the deliberateness in the making of those lines," Van Gelder explains. These marks weren't just the product of random swipes of a tool across the surface of an old bone; they were created with thoughtfulness, she says.

Who exactly made these scratches, however, remains a mystery. Neanderthals likely didn't venture this far into Asia; to date, we only know of Neanderthals making it as far east as Denisova cave in the Altai mountains—nearly 1,900 miles northwest of where the etched fragments were found. And it's unclear if modern humans made it this far north at this early date.

The two fossil skulls found at the site contain a mosaic of ancient and modern features. Prior work hinted that they might be Denisovan, but thanks to the scant traces of Denisovan remains yet found, DNA evidence would be necessary to say for sure. Past research also suggested that Denisovans could be responsible for personal adornments, such as tooth pendants, found in Denisova cave. Still, scientists can't exclude the possibility that modern humans had a hand in making those artifacts.

"My take on this is: It wasn't purely modern humans," University of Wisconsin-Madison paleoanthropologist John Hawks says of the capacity for crafting such material culture. "In fact, the idea that anything was 'pure' has gone by the wayside."

The more scientists look, the more interspecies mingling they seem to find. Genetic evidence shows that when waves of modern humans began pouring out of Africa around 60,000 years ago, they met and interbred with at least two of their hominin cousins: Neanderthals and Denisovans. Along with this genetic exchange, there could have been a cultural exchange, too. (Learn more about how multiple lines of ancient humans interbred with us.)