Carelessly discarded chewing gum is a nuisance when fresh, but it might become a scientific treasure—if it sticks around long enough. About 5,700 years should do. That’s the age of a lump of birch pitch recently found in Lolland, Denmark, the site of an archeological dig.
Birch pitch, the Juicy Fruit of ancient peoples going back to the Paleolithic period, is often found to have been chewed. And the Lolland birch pitch, a well-masticated sample, was left behind by someone who enriched it with their DNA. This DNA has now been sequenced to yield a complete human genome.
The genome, which was assembled by scientists based at the University of Copenhagen, belonged to a female who likely had dark skin, dark brown hair, and blue eyes. The scientists decided to name her Lola in commemoration of the place, Lolland, where her birch pitch was found.
Lola, unlike today’s gum chewers, didn’t chew to amuse herself or freshen her breath. She was probably preparing birch pitch, a black-brown substance produced by heating birch bark, to be used as an adhesive. After hot, soft birch pitch cools, it can, upon chewing, become malleable again, fitting it for use in hafting, or the joining of tool components such as wooden handles and sharpened stones.
Leaving a sticky mess behind was the farthest thing from Lola’s mind. She was doing something that was socially useful in her day. Even better, she performed a useful service, albeit unknowingly, for future generations. Not only did she bequeath us her own DNA—both nuclear and mitochondrial—she preserved DNA from her oral microbiome, as well as DNA from several important pathogens.
The scope of Lola’s contributions is described in an article (“A 5700-year-old human genome and oral microbiome from chewed birch pitch”) that appeared December 17 in the journal Nature Communications.
“We sequence the human genome to an average depth of 2.3× and find that the individual who chewed the pitch was female and that she was genetically more closely related to western hunter-gatherers from mainland Europe than hunter-gatherers from central Scandinavia,” the article’s authors detailed. “In addition, we identify DNA fragments from several bacterial and viral taxa, including Epstein-Barr virus, as well as animal and plant DNA, which may have derived from a recent meal.”
These results, which highlight the potential of chewed birch pitch as a source of ancient DNA, represent the first time that an entire ancient human genome has been extracted from anything other than human bones.
While working with the birch pitch, the scientists evaluated several methods for sample preparation. The best method in terms of endogenous human DNA content, the scientists found, used a proteinase K–based lysis buffer to extract DNA, which was then purified with phenol-chloroform. However, when this method was used for metagenomic profiling, the extracts were found to be contaminated with Delftia spp., a known laboratory contaminant. Consequently, contaminated libraries were excluded from metagenomic profiling.
Once the metagenomic profiling was performed, it yielded interesting results, including many commensal species and opportunistic pathogens.
“The preservation is incredibly good, and we managed to extract many different bacterial species that are characteristic of an oral microbiome,” noted Hannes Schroeder, the current paper’s senior author and an associate professor at the University of Copenhagen’s Globe Institute. “Our ancestors lived in a different environment and had a different lifestyle and diet, and it is therefore interesting to find out how this is reflected in their microbiome.”
Schroeder and colleagues also found DNA that could be assigned to Epstein-Barr Virus, which is known to cause infectious mononucleosis or glandular fever. According to Schroeder, ancient “chewing gums” bear great potential in researching the composition of our ancestral microbiome and the evolution of important human pathogens.
“It can help us understand how pathogens have evolved and spread over time, and what makes them particularly virulent in a given environment,” he continued. “At the same time, it may help predict how a pathogen will behave in the future, and how it might be contained or eradicated.”
Although birch pitch can tell us a lot about pathogens, many of us are probably more interested in what it can tell us about our ancestors. For example, Lola’s birch pitch gives us additional information about her people, who were exploiting wild resources well into the Neolithic, a time when farming and domesticated animals were first introduced into southern Scandinavia.
Analysis of Lola’s birch pitch revealed traces of plant and animal DNA—specifically hazelnuts and duck—which may have been part of Lola’s diet.
“We have shown that pieces of chewed birch pitch are an excellent source of ancient human and nonhuman DNA,” the article’s authors concluded.” In the process of chewing, the DNA becomes trapped in the pitch where it is preserved due to the aseptic and hydrophobic properties of the pitch which both inhibits microbial and chemical decay. The genomic information preserved in chewed pieces of birch pitch offers a snapshot of people’s lives, providing information on genetic ancestry, phenotype, health status, and even subsistence.”
