The people who lived at Huseby-Kiev in western Sweden 10,000 years ago made their living by hunting and fishing. That doesn't sound surprising until you consider that this was a landscape that had, until recently, been covered by ice sheets 4km (2.5 miles) thick. How they occupied the re-emerging landscape is a bit of a mystery. We don't know much about who they actually were, where they came from, or how they made their way into Sweden as the ice receded.

In the 1990s, archaeologists recovered a few chewed-up lumps of birch bark pitch, some of which still held fingerprints and tooth marks left behind from millennia ago. Using this ancient chewing gum, archaeologist Natalija Kashuba of Uppsala University recently recovered DNA from two women and one man who had lived, worked, and apparently chewed gum on the shores of ancient Sweden. That means we can now link DNA from ancient people to their artifacts, and that's a big clue about how people migrated into Scandinavia after the Ice Age.

Two groups of hunter-gatherers met in Sweden

Birch bark pitch, like other saps and resins from various trees around the world, makes a decent chewing gum. When chewed and softened, it's also a handy glue for repairing cracked pottery or gluing bone points onto stone blades to make a vicious-looking composite point (see gallery). That's how people at Huseby-Kiev seem to have used it.

Based on the tools and other hints that these people also left behind, it seems that people converged on Scandinavia from two directions as the ice sheets receded. One group migrated northward from western Europe, while another migrated southwest from the plains of modern-day Russia. These two groups of people each had their own unique ways of making stone tools, which is how archaeologists have managed to tell their sites apart and trace their migration paths.

The people who came from Russia, for example, brought a technology called pressure flaking, which involves using a pointed stick or bone to break off small flakes from the edge of a stone tool, creating a sharp blade. Over time, this new eastern pressure-flaking technology eventually replaced the older western European techniques.

When these two populations of hunter-gatherers met in Scandinavia, they seem to have intermarried. Over time, the mixing of their gene pools created a new population, which anthropologists call Scandinavian Hunter-Gatherers (anthropologists are not widely known for creative naming schemes). We know this from DNA from human remains several centuries younger than the site at Huseby-Kiev.

The gum at Huseby-Kiev is the oldest human DNA ever recovered from Scandinavia, and it sheds some light on the time when these populations were first encountering each other.

Kashuba et al. 2019

Kashuba et al. 2019

Under the gum

When Kashuba and her colleagues compared the DNA from all three pieces of chewing gum to databases of ancient DNA from other sites, it turned out that the two women and the man from Huseby-Kiev were closely related to the Scandinavian hunter-gatherer group—but their genomes looked more like Mesolithic people from western Europe than from Russia. It's the first time archaeologists have found Scandinavian hunter-gatherer DNA clearly linked with stone tools, and it shows that people in Scandinavia 10,000 years ago were already using the newer eastern European method of pressure-flaking.

It also shows that the spread of the new technology wasn't just carried by people from eastern Europe. The two groups were trading ideas, not just genes.

On a smaller scale, the DNA samples in the three unassuming lumps of pitch reveal something about the lives and culture of people 10,000 years ago. The tooth marks in the gum came from deciduous teeth (which most people call baby teeth), suggesting that making stone tools wasn't strictly adult work. And two of the three genomes were genetically female, which suggests that tool-making also wasn't a gender-specific job.

As DNA sequencing technology improves, archaeologists are finding ancient DNA in surprising places. Earlier this year, the stem of a clay pipe revealed the genome of an enslaved woman who once lived in Maryland. Kashuba and her colleagues say that gums, resins, and similar materials from around the world may also be good sources of ancient DNA, even in places where few human bones have managed to preserve DNA from the distant past. They also suggest that these materials may hold proteins and other molecules which could offer clues about ancient people's diets and microbiomes.

Communications Biology, 2018. DOI: 10.1038/s42003-019-0399-1 (About DOIs).