5,700 years ago, a woman in what is now Denmark chewed a lump of birch-bark pitch for a while and then dropped it. Millennia later, the DNA she left behind reveals her entire genome, a census of the bacteria living in her mouth, and a few hints about a recent meal.
If you’re a hunter-gatherer who needs to haft a stone tool, birch-bark pitch makes a handy adhesive, but you might have to chew it to make it pliable enough to work with. Pitch is watertight and contains an antiseptic compound called betulin, so it’s great at preserving DNA. In fact, archaeologists in Scandinavia have found more human DNA in bits of chewed pitch than they have in skeletons (which have been relatively rare at prehistoric Scandinavian sites we’ve studied thus far).
But this is the first time researchers have managed to sequence an ancient person’s whole genome from a lump of chewed pitch. Only about a third of the DNA bioarchaeologist Hannes Schroeder, of the University of Copenhagen, sampled was human; the other 68 percent came from the ancient woman’s microbiome and traces of a prehistoric meal. This single discarded piece of ancient chewing gum tells us that the ancient woman, who Schroeder and his colleagues have nicknamed Lola, was probably lactose intolerant, ate duck and hazelnuts, and may recently have had pneumonia. She also had blue eyes, dark brown hair, and dark skin.
“This combination of physical traits has been previously noted in other European hunter-gatherers, suggesting that this phenotype was widespread in Mesolithic Europe and that the adaptive spread of light skin pigmentation in European populations only occurred later in prehistory,” wrote Schroeder and his colleagues.
She lived through the Neolithic revolution
The detail we can extract with DNA turns a seriously unprepossessing artifact into a deeply personal link to life in the past. It also sheds light on the much larger cultural upheaval that was reshaping Lola’s world. While Lola was eating hazelnuts at the prehistoric settlement now called Syltholm, people from southwest Asia were spreading westward across Europe, bringing a new way of life with them: farming.
Around the time Lola lived in Denmark, the hunter-gatherer culture called Erlebolle gave way to the Funnel Beaker culture, a way of life built on farming and livestock. From archaeological data, we know that cultural change happened very quickly in Denmark, with people’s settlement patterns and ways of making a living changing almost overnight. But what we don’t know is whether that’s because new people moved in, displacing the old Erlebolle hunter-gatherers, or if it’s because the people already living in Denmark adopted new ideas.
Lola’s genome suggests that she’s not related to the Neolithic farmers who swept westward. And that implies that the newcomers didn’t immediately replace or absorb the hunter-gatherers who had been living in Denmark since the end of the Ice Age. In Lola’s day, it seems that at least a few of the old hunter-gatherer groups were still around, even if they’d adopted new ways of life. That lines up well with other genomic evidence from elsewhere in Europe, but it’s important not to draw firm conclusions based on a single genome.
Duck and hazelnut were on the menu
But what was Lola’s life like during this time of cultural change? A small percentage of the DNA in the pitch came from other things she had recently chewed: hazelnut and a species of mallard, bits of which must still have been in her mouth when she picked up the pitch. That’s potentially a more detailed image than you wanted, but it suggests that even after Lola’s community took up farming, they still relied on wild foods, too. Duck bones and hazelnuts at the site back up the DNA evidence.
Lola’s genome also contained alleles that are usually associated with the inability to produce lactase—an enzyme that’s important in digesting milk—as an adult. If lactose intolerance was still common among people in Scandinavia just after the Neolithic revolution (the wave of migration and cultural change that introduced agriculture to most of Europe), then Europeans probably evolved the ability to drink milk as adults relatively recently, after dairy farming became a common way of life. DNA evidence from other sites supports that idea, too.
A Neolithic case of pneumonia?
Most of the DNA recovered from the lump of pitch belonged to bacteria and viruses. At other sites, archaeologists have gotten some information about ancient microbiomes from DNA and proteins preserved in calculus—hardened dental plaque still clinging to the teeth of ancient skeletal remains. But calculus is a long-term record of the tiny organisms that live in your mouth; the DNA Lola left behind in her chewing gum is more like a snapshot.
Overall, Lola’s oral microbiome looked a lot like modern ones (there are whole datasets of oral microbiomes for comparison). Other than a few bacteria associated with periodontal disease, the microbes in Lola’s mouth were harmless, and that’s still the case for modern people. Schroeder and his colleagues did find some genetic sequences from the Epstein-Barr virus, which is something Lola would have in common with about 90 percent of adults in the world today. It’s likely that the virus never made her sick; most people catch it as children and never experience symptoms (occasionally Epstein-Barr virus can cause a serious fever or increase a person’s risk of Hodgkin’s lymphoma and some autoimmune diseases later in life).
Lola may have been recovering from a case of pneumonia on the day she chewed the birch-bark pitch, because genetic sequences from the bacterium Streptococcus pneumoniae showed up in her microbiome. Schroeder and his colleagues found several sets of genes called virulence factors, traits associated with the pathogen’s ability to infect a host and thwart the immune system. S. pneumoniae has a knack for picking up virulence factors from other related bacteria, so ancient DNA may help researchers trace its evolutionary history.
Hints about an even more ancient past
Lola’s genome also tells archaeologists about the first people to move into Scandinavia after the ice sheets retreated between 12,000 and 11,000 years ago, generations before she lived at Syltholm.
Based on other archaeological and genetic evidence, we know that after the ice’s retreat, people from different areas of Eurasia moved into Scandinavia along two routes. Some hunter-gatherers from mainland Europe moved northward through Denmark, while others from farther northeast moved southward along the coast of Norway. Lola is much more closely related to the group from mainland Europe—in fact, her genome carries no traces of ancestry from the second group at all.
Schroeder and his colleagues say Lola’s DNA supports the current theory that the people moving in from the northeast (archaeologists creatively call them Eastern Hunter-Gatherers) hadn’t reached southern Denmark by 5,700 years ago. But that idea, Schroeder and his colleagues admit, must be taken with a grain of salt, because we only have one genome to work with.
As always, we need more data, but for now, researchers have something to chew on.
Nature Communications, 2020 DOI: 10.1038/s41467-019-13549-9 (About DOIs).