What Chewed-Up Gum Reveals About Life in the Stone Age

Chewed tar is an unexpectedly great source of ancient DNA.

A piece of chewed birch-bark tar (Natalija Kashuba)

No one today quite understands how they did it, but people in the Stone Age could turn ribbons of birch bark into sticky, black tar. They used this tar to make tools, fixing arrowheads onto arrows and blades onto axes. And they chewed it, as evidenced by teeth marks in some lumps.

These unassuming lumps of chewed birch-bark tar turn out to be an extraordinary source of ancient DNA. This month, two separate research groups posted preprints describing DNA from the tar in Stone Age Scandinavia. The two papers have not yet been peer reviewed, but they are already generating excitement about what they herald.

“It’s really amazing,” says Pontus Skoglund, an ancient-DNA researcher at Francis Crick Institute who was not involved with either study. Ancient DNA from human bones and teeth have recently revolutionized the study of the past, but many cultures over time did not bury their dead and left no remains to analyze. Chewing gum could fill in some of the gaps. It could also reveal a wealth of additional information, such as who helped make the Stone Age tools, what they ate, and what bacteria lived in their teeth.

The first new paper describes human DNA from three 10,000-year-old pieces of birch-bark tar, all found at a site called Huseby Klev in western Sweden. Having never extracted DNA from tar before, the team tried a protocol originally designed to extract DNA from feces—and it worked. Each piece of tar appears to have been chewed by just one person. In total, the tar pieces captured the DNA of two females and one male.

The site where the lumps were found was littered with the raw material and leftovers from making stone blades. From this, the authors suggest that it was actually a site for making tools, and chewing birch-bark tar was a step in the production process. If so, it would mean both men and women made tools during the Stone Age. And because some of the teeth marks appear to be from baby teeth, it suggests that children had a role, too. All this hints at the social structure of Stone Age society. “The most exciting part is how close we can come to the culture,” says Natalija Kashuba, a researcher at the University of Oslo and first author on the paper.

Ancient DNA can be prone to contamination from modern humans handling the sample. In this case, however, the DNA in the chewing gum appears to be genuinely ancient. “It’s very clear the DNA they get out has ancestry that was only there around 8,000 or 5,000 years ago, and it’s not really there anymore,” Skoglund says. The DNA of the three people looked a lot like the DNA of other hunter-gatherers who lived in northern Europe around that time.

In a second paper, researchers predicted the physical appearance of a 5,700-year-old woman based on chewed birch-bark tar found in Denmark. To modern eyes, she would have looked unusual. She had dark skin, dark hair, and blue eyes, all of which were characteristic of Europe’s hunter-gatherers then. A recent reconstruction of Cheddar Man, a 10,000-year-old hunter-gatherer skeleton found in Britain, also revealed dark skin and blue eyes.

The second team analyzed nonhuman DNA in the chewing gum, too. “You can recover microbial DNA,” says Hannes Schroeder, an ancient-DNA researcher at the University of Copenhagen who led the study. “And this opens up whole other possibilities.” Other groups have studied ancient plaque that built up over years on teeth, but the chewing gum provides an instant snapshot of what was in the mouth. It’s comparable to how scientists study the oral microbiome of people today, and unsurprisingly, the microbial species they found were broadly similar. There were differences in two types of microbes called Veillonellales and Neisseriales, which the team suggests may be due to hunter-gatherers eating fewer carbohydrates before the advent of farming.

They also found direct evidence of what the woman ate, in the form of DNA from mallard duck and eel in the chewed tar. This matches up with archaeological evidence at the site, including duck bones and tools for catching eels.

In the Stone Age, the area where this woman was chewing and spitting out birch-bark tar was a lagoon. Today, it’s the site of a massive construction project for a tunnel connecting Denmark and Germany. Theis Jensen, a graduate student in Schroeder’s research group, had helped with pre-construction excavations, and he was the one who convinced colleagues at Museum Lolland-Falster, which is responsible for the excavations, to hand over a piece of birch-bark tar to test. Using the DNA contained within, the team has been able to piece together what one 5,700-year-old woman looked like, what she ate, and even the microbes living in her mouth. “It’s like having the ghost in front of you,” Jensen says.

For all the questions it could answer, birch-bark tar is still at the center of many mysteries. Making it requires a steady application of heat and an oxygen-free environment, and archaeologists aren’t quite sure how people in the Stone Age achieved this without ceramic pots. It’s also unclear exactly why they chewed it: Was it for recreation or health or for making tools or all of the above? Whatever it was, the people who chewed birch-bark tar unwittingly left behind a rich record of the past.