by About 9,700 years ago on a harvest day, a group of people were camping on the west coast of Scandinavia. They were hunters who fished, hunted and gathered resources in the area.
Some teenagers, both boys and girls, were chewing resin to produce glue, just after eating trout, deer and hazelnuts. Due to a severe gum infection (periodontitis), one of the teenagers had problems eating the chewy deer, as well as preparing the resin by chewing them.
This picture of the Mesolithic period, just before Europeans started farming, comes from an analysis of DNA left in the chewing resin we have made, now published in Scientific Reports.
The site is now known as Huseby Klev, located north of Gothenburg (Göteborg), Sweden. Archaeologists excavated it in the early 1990s, and found around 1,849 flint artifacts and 115 pieces of resin (mastic). The site was radiocarbon dated to between 10,200 and 9,400 years ago, and one of the pieces of resin was dated to 9,700 years ago.
Some of the resin has teeth marks, indicating that children, teenagers in fact, had chewed on them. Mastic lumps, often with impressions of teeth, fingerprints or both, are not found on Mesolithic sites.
The resin pieces we analyzed were made from birch bark pitch, which is known to have been used as an adhesive in stone tool technology from the Middle Palaeolithic onwards. However, they are also chewed for recreational or medicinal purposes in traditional societies.
A variety of substances with similar properties, such as resins from coniferous trees, natural bitumen, and other plant gums, are known to be used in analogous ways in many parts of the world.
The power of DNA
In some of the resin, half of the DNA extracted was of human origin. This is quite a lot compared to what we often find in ancient bones and teeth.
I show some of the oldest human genomes from Scandinavia. It has a distinct ancestry profile that is common among the Mesolithic hunter-gatherers who once lived there.
Some of the resin contains male DNA while others contain female DNA. We think teenagers of both sexes were preparing glue for use in tool making, such as attaching a stone ax to a wooden handle.
But where was the other half of the DNA of non-human origin? Most of this DNA comes from organisms such as bacteria and fungi that have lived in the mast since it was discarded 9,700 years ago. But it was from bacteria that lived in the person who chewed it, as well as material that the person was chewing on before they put the birch bark field in their mouth.
Analyzing all this DNA is a challenging task and takes a new direction. We had to adapt the existing computing tools and also develop some new analysis strategies. Therefore, this work is now a starting point for developing a new workflow for this type of analysis.
This includes mining the DNA using different strategies to characterize it, trying to assemble short DNA fragments into longer ones and using machine learning techniques to work out which DNA fragments belong with pathogens (harmful micro-organisms). It is also about comparing the data with what we see in the mouths of modern people with tooth decay (caries) and periodontitis.
Higher organisms
Naturally, we found the type of bacteria one would expect in an oral microbiome, the range of naturally occurring microorganisms found in the mouth. We also found traces of bacteria associated with conditions such as tooth decay or caries (Streptococcus mutants), and systemic diseases such as Hib disease and endocarditis. There were also bacteria that could cause abscesses.
Although these pathogenic microorganisms were present at an elevated frequency, they were not clearly above the level expected for a healthy oral microbiome. Therefore there is no conclusive evidence that members of the group suffered from diseases associated with these microorganisms.
What we found, however, was an abundance of bacteria associated with serious gum disease – periodontitis. When we applied a machine learning strategy (in this case, a technique called Random Forest modeling) we concluded that the girl who chewed one of the pieces of resin probably suffered from periodontitis – with a probability of more than 75%.
We also found DNA from more organisms than just bacteria. We found DNA for red deer, brown trout and hazelnuts. This DNA probably came from material the teenagers were chewing before putting the birch bark in their mouths.
However, we have to be a bit careful because what we find also depends on the comparison data we have. Because genomes from eukaryotic organisms – the group that includes plants and animals – are larger and more complex than those from microorganisms, it is more difficult to assemble a high-quality eukaryotic genome.
The resin samples contain fewer eukaryotic genomes, and are of lower quality. This means that our brown trout, for example, may not be a brown trout, but at least we feel certain that it is from the salmon family.
We also found a lot of fox DNA, but this is more difficult to interpret. Fox meat may have been part of the diet, but these teenagers could have been chewing on tendons and fur from foxes for use in textiles. Alternatively, the fox DNA could even be from territorial marking and got into the resin after it was spit out.
However, what we have learned is certainly a big step towards understanding these fascinating records of human culture from the Stone Age. As we analyze more of these, even more surprises may emerge.
This article from The Conversation is republished under a Creative Commons license. Read the original article.
Anders Götherström receives funding from: Swedish Research Council (2019-00849_VR), Riksbankens Jubileumsfond (P16-0553:1)
Emrah Kırdök does not work for, consult with, own shares in, or receive funding from any company or organization that would benefit from this article, and has not disclosed he has no relevant connections beyond his academic appointment.
