The site is called Huseby Klev. It sits on the island of Orust on the western coast of Sweden, about an hour’s drive north of Gothenburg. Approximately ten thousand years ago, when sea levels were higher and the coastline ran further inland, Huseby Klev was a coastal fishing camp at the end of a narrow fjord. The people who lived there were hunter-gatherers, part of the first wave of human populations to settle Scandinavia after the retreat of the Weichsel ice sheet. They were also, by the available evidence, regular chewers of birch bark pitch.

Birch bark pitch is the tar-like black resin produced by heating birch bark in low-oxygen conditions. It hardens at room temperature and becomes pliable when warmed in the mouth, which is what made it useful as glue for the production of stone tools, weapons, and hafted implements. The chewing was likely the practical step in the manufacturing process. The hunter-gatherers of Huseby Klev produced birch tar, chewed it to soften it, used the softened mass as adhesive, and discarded the lumps that were no longer needed.

The discarded lumps preserved more than the chewers could have imagined.

The site and what was found

Huseby Klev was excavated in the early 1990s by a team of Swedish archaeologists. The site preserved a remarkable density of organic material, including human bones, animal bones, fish bones, wooden tools, lithic tools, and approximately ninety separate pieces of masticated birch bark pitch. The exceptional preservation was caused by a layer of marine clay that had sealed the site shortly after it was abandoned, creating what the archaeological team described as a geological time capsule. The clay kept the organic material wet, anaerobic, and protected from the bacterial decomposition that would have destroyed it within decades at most other Mesolithic sites in Europe.

The pitch pieces were of particular interest to the excavators because many of them carried clearly visible tooth marks. Six of the pieces were cast in plaster for forensic analysis. The osteological work on the casts indicated that all six pieces had been chewed by people younger than twenty years of age. Three of the chewers had been children between five and eleven years old. Three had been teenagers. Both male and female adolescents were among the chewers.

In the 1990s, the analytical technology that could recover and sequence ancient DNA from non-bone material did not yet exist. The chewed pitch pieces were carefully stored under the guardianship of the Swedish archaeologist Bengt Nordqvist, with the understanding that future generations of scientists might find uses for them that the excavators could not predict. The wait was approximately thirty years.

What the 2019 study found

In May 2019, a research team led by Natalija Kashuba at the Museum of Cultural History in Oslo, with collaborators at Stockholm University and Uppsala University, published a paper in Communications Biology reporting the successful extraction and sequencing of human DNA from three of the Huseby Klev pitch pieces. The pieces were directly carbon-dated to between 9,540 and 9,880 calibrated years before present, which is approximately 9,700 years ago in everyday language.

The team generated genome-wide data for three individuals from the three pieces, two females and one male. The DNA had survived in sufficient quantity that the team could examine population-level genetic affinities. The three individuals showed close genetic relationships with other Mesolithic Scandinavian hunter-gatherers and with the broader Western Hunter-Gatherer population that had spread across Ice Age Europe. The pitch pieces became, at the time of publication, the oldest sources of human DNA ever recovered from Scandinavia.

The mechanism of preservation, which the Kashuba team examined in some detail, came down to the chemistry of the birch tar itself. Birch bark pitch contains betulin, lupeol, and other terpenoid compounds that have natural antimicrobial properties. When the chewed pitch was discarded, the antimicrobial chemistry of the tar slowed or prevented the bacterial decomposition that would normally destroy DNA within decades. The pitch effectively encapsulated and chemically protected the saliva, cells, and oral microbiome of each individual chewer at the moment the pitch was spat out, and the marine clay sealing the site preserved the protected material for the next ten thousand years.

What had begun, in the early 1990s, as an effort to find conventional Mesolithic artefacts at a Swedish coastal site had ended with the complete genomic reconstruction of three teenagers and children who had lived and chewed at the site approximately a hundred centuries earlier.

What the 2024 follow-up revealed

In January 2024, a second team led by Emrah Kırdök at Mersin University in Turkey, again with Kashuba and Anders Götherström as co-authors, published a follow-up study in Scientific Reports. The 2024 paper used the same three pitch pieces from the 2019 study but applied a different analytical approach. Rather than focusing on the human DNA, the team examined the non-human DNA that had been preserved alongside it. The pitch pieces, on the new analysis, turned out to contain extensive genetic material from the microorganisms that had lived in the chewers’ mouths and from the plants and animals that had recently passed through them.

The diet, on the genetic evidence, was consistent with what would be expected for Mesolithic coastal hunter-gatherers. The Kırdök team identified DNA sequences from red deer, brown trout, mallard duck, European turtle dove, hazelnut, and European crab apple. They also identified DNA from wolf, red fox, and arctic fox, suggesting either that these canids had been hunted for fur and meat or that their remains had been processed near where the pitch was chewed. Most striking, the team identified DNA sequences from mistletoe, a plant which is known from later archaeological contexts to have been used by Mesolithic Europeans to produce poison for arrowheads.

The oral microbiome data was more sobering. The bacterial DNA in the pitch pieces showed substantial overrepresentation of species that are now known to be associated with periodontitis, including Treponema denticola, Porphyromonas gingivalis, Actinomyces johnsonii, and Streptococcus anginosus. The 2024 team applied two independent statistical methods to estimate the probability that the chewers had been suffering from periodontitis. Both methods returned probabilities above 70 per cent. For one sample, the probability of an active periodontitis-like oral microbiome was 84 per cent.

The teenagers, in other words, had bad gums.

What this tells us about Mesolithic life

The combined evidence from the 2019 and 2024 studies produces an unusually detailed picture of three individuals from a population whose lives have been otherwise inaccessible to history. Three teenagers or younger children, living at a coastal fishing camp in what is now western Sweden, ate a diet of red deer, freshwater trout, duck, hazelnuts, and apples. They were involved, at least peripherally, in the processing of fox, wolf, and arctic fox carcasses. They handled mistletoe, which their community may have used to poison arrowheads. They worked with birch bark pitch as part of the production of stone tools and weapons. They had visibly poor oral health, with gum disease that would, in modern clinical practice, prompt urgent dental intervention.

The poor oral health, in the view of the Kırdök team, is consistent with what is known about Mesolithic dental practices more broadly. Hunter-gatherer populations did not have dental care in any modern sense. They used their teeth for tool work, including stripping fibres, softening leather, and shaping bone and antler implements. The wider use of teeth as tools likely increased the bacterial load on the oral microbiome and made periodontitis more common. The teenagers at Huseby Klev were not exceptionally unhealthy by Mesolithic standards. They were typical.

The diet, similarly, is consistent with the archaeological record from the site itself. Animal bones recovered from the Huseby Klev excavations include red deer, brown trout, wolf, fox, and various waterfowl. Plant remains include hazelnut shells and apple cores. The DNA evidence from the chewed pitch confirms, at the level of individual chewers on particular days, what the bone-and-shell archaeology had already established at the level of the broader population.

The broader scientific significance

The Huseby Klev chewed pitch is now one of the strongest sources of Mesolithic human DNA in Europe. The technique that the Kashuba team demonstrated in 2019 has since been applied to other ancient pitch finds. A 2019 study by Theis Jensen and colleagues at the University of Copenhagen, published in Nature Communications, sequenced a complete human genome from a 5,700-year-old piece of chewed birch pitch found at the Syltholm site in southern Denmark. The Danish individual, whom the researchers nicknamed Lola, was a dark-skinned, blue-eyed female who had recently eaten hazelnuts and duck. The implication of both studies, taken together, is that ancient chewed pitch is a reliable and underused source of ancient genetic material that can extend the reach of population genetics into time periods and populations where bone preservation has been poor.

The implication for archaeology generally is more interesting than the implication for any single individual study. For most of the history of archaeogenetics, the recovery of ancient human DNA has depended on the survival of bone and tooth tissue. Bones survive only under certain soil and climate conditions, and many ancient populations are effectively invisible to the modern genetic record because the bones of their dead have decomposed. Chewed pitch survives under different conditions than bone does. It is preserved by anaerobic burial in clay and protected by its own antimicrobial chemistry, which means it can preserve DNA in environments where bone would not. The Mesolithic populations of southern Scandinavia, who left few well-preserved cemeteries, are now reachable through their chewing gum in ways they were not reachable through their burials.

What it means

Three teenagers chewed wads of birch bark pitch at a fishing camp on the western coast of Sweden approximately ten thousand years ago. They were processing the pitch as part of the production of tools or weapons. They were also, at the moment of chewing, recently fed, slightly unwell, and aware of nothing about what the act of chewing would eventually disclose.

Their saliva entered the pitch, was sealed in clay, and lay undisturbed under several metres of sediment for ten thousand years.

It was excavated in the 1990s, stored in a Swedish museum collection, and analysed with technology that did not exist until decades after the excavation. The complete genomes of all three chewers have now been sequenced. Their diet on the day of chewing has been reconstructed from the plant and animal DNA in their mouths. The bacterial signature of their gum disease has been catalogued.

The information density of a discarded wad of Stone Age chewing gum, on the strongest current evidence, is genuinely remarkable. The same is true for the approximately ninety other pitch pieces from the same site that have not yet been analysed.

The chewing was a moment of distraction in a coastal fishing camp ten thousand years ago. The wad was thrown away. The clay closed over it.

It turned out to be the wrong moment to be distracted.