About 9,700 years ago, a group of people camped on the west coast of Scandinavia on an autumn day. They were hunter-gatherers who had fished, hunted and gathered resources in the area.
Some teenagers, both boys and girls, chewed resin to produce glue just after eating trout, deer and hazelnuts. Due to severe gum disease (periodontal disease), one of the teenagers had problems eating the tough venison and preparing the resin by chewing it.
This snapshot of the Mesolithic period, just before Europeans began farming, comes from the analysis of the DNA left in the chewed resin, now published in Scientific Reports.
The location is now known as Huseby Klev, located north of Gothenburg (Gothenburg), Sweden. It was excavated by archaeologists in the early 1990s and yielded some 1,849 flint artefacts and 115 pieces of resin (mastic). The site has been radiocarbon dated to between 10,200 and 9,400 years ago, while one of the resin pieces is dated to 9,700 years ago.
Some of the resin has teeth marks, indicating that children, actually teenagers, have chewed it. Chewed cusps, often with impressions of teeth, fingerprints or both, are not uncommon to find at Mesolithic sites.
The resin pieces we analyzed are made from birch bark pitch, which is known to have been used as an adhesive in stone tool technology from the Middle Paleolithic onwards. However, in traditional societies they were also chewed for recreational or medicinal purposes.
A variety of substances with similar properties, such as conifer resins, natural bitumen and other vegetable gums, are known to be used in analogous ways in many parts of the world.
The power of DNA
In one part of the resin, half of the extracted DNA was of human origin. This is a lot compared to what we often find in old bones and teeth.
I represents some of the oldest human genomes from Scandinavia. It has a particular ancestry profile common among Mesolithic hunter-gatherers who once lived there.
Some of the resin contains male human DNA, while others have female DNA. We think that teenagers of both sexes were preparing glue for use in tool making, such as attaching a stone ax to a wooden handle.
But what about the other half of the DNA, which was of non-human origin? Most of this DNA comes from organisms such as bacteria and fungi that have lived in the mastic since it was discarded 9,700 years ago. But some of it came from bacteria that lived inside humans and chewed it, along with material that humans had chewed before putting the birch bark pitch in their mouths.
Analyzing all this DNA is a demanding task and breaks new ground. We had to both adapt existing computational tools and also develop some new analytical strategies. As such, this work has become the 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 piece together short DNA fragments into longer ones, and using machine learning techniques to figure out which DNA fragments make up pathogens ( harmful micro-organisms). It also involves comparing the data with what we see in the mouths of modern people with tooth decay (caries) and periodontal disease.
Higher organisms
Naturally, we found the kinds of bacteria you would expect in an oral microbiome, the range of naturally occurring microorganisms found in the mouth. We also found traces of bacteria involved in conditions such as tooth decay or caries (Streptococcus mutans), and systemic diseases such as Hib disease and endocarditis. There were also bacteria that can cause abscesses.
Although these pathogenic microorganisms were present at an increased frequency, they were not clearly above the levels expected for a healthy oral microbiome. Thus, there is no conclusive evidence that members of the group suffered from diseases associated with these microorganisms.
What we did find, however, was an abundance of bacteria linked to serious gum disease: periodontal disease. 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 periodontal disease – with a probability of more than 75% .
We also found DNA from larger organisms than just bacteria. We have found DNA for red deer, brown trout and hazelnuts. This DNA likely came from material the teens had chewed before putting the birch pitch in their mouths.
However, we have to be a little careful, because what exactly we find also depends on the comparison data we have. Because genomes of eukaryotic organisms – the group to which plants and animals belong – are larger and more complex than those of microorganisms, it is more complicated to assemble a high-quality eukaryotic genome.
There are fewer eukaryotic genomes in the resin samples and they are of lower quality. This means that, for example, our brown trout may not actually be a brown trout, but we can at least be sure that it belongs to 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 would also have chewed fox tendons and fur for use in textiles. Alternatively, the fox DNA could even have come from territorial marking and ended up in the resin after it was spit out.
What we have learned, however, certainly represents a major step in understanding these fascinating accounts of Stone Age human culture. As we analyze more of this, even more surprises may emerge.
This article is republished from The Conversation under a Creative Commons license. Read the original article.
Anders Götherström receives funding from: the 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 disclosed no relevant affiliations beyond their academic appointment.