Human Genome Recovered from 5,700-Year-Old Chewing Gum

History
سه شنبه 26 آذر 1398 ساعت 17:24
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Smith sonian mag
Modern chewing gums, which often contain polyethylene plastic , could stick around for tens or even hundreds of years, and perhaps much longer in the right conditions. Some of the first chewing gums, made of birch tar and other natural substances, have been preserved for thousands of years, including a 5,700-year-old piece of Stone Age gum unearthed in Denmark.
For archaeologists, the sticky stuff’s longevity can help piece together the lives of ancient peoples who masticated on the chewy tar. The ancient birch gum in Scandinavia preserved enough DNA to reconstruct the full human genome of its ancient chewer, identify the microbes that lived in her mouth, and even reveal the menu of a prehistoric meal.
“These birch pitch chewing gums are kind of special in terms of how well the DNA is preserved. It surprised us,” says co-author Hannes Schroeder , a molecular anthropologist at the University of Copenhagen, Denmark. “It’s as well-preserved as some of the best petrous [skull] bones that we’ve analyzed, and they are kind of the holy grail when it comes to ancient DNA preservation.”
Birch pitch, made by heating the tree’s bark, was commonly used across Scandinavia as a prehistoric glue for attaching stone tools to handles. When found, it commonly contains toothmarks. Scientists suspect several reasons why people would have chewed it: to make it malleable once again after it cooled, to ease toothaches because it’s mildly antiseptic , to clean teeth, to ease hunger pains, or simply because they enjoyed it.
The gum’s water-resistant properties helped to preserve the DNA within, as did its mild antiseptic properties which helped to prevent microbial decay. But the find was also made possible by the conditions at the site, named Syltholm, on an island in southern Denmark, where thick mud has perfectly preserved a wide range of unique Stone Age artifacts. Excavations began at the site in 2012 in preparation for the construction of a tunnel, affording the Museum Lolland-Falster a unique chance for archaeological field work.
No human remains have yet been found at Syltholm—unless you count the tiny strands of DNA preserved in the ancient gum Schroeder and colleagues described today in Nature Communications .
The discarded gum yielded a surprising amount of information about its 5,700-year-old chewer. She was a female, and while her age is unknown, she may have been a child considering similar birch pitch gums of the era often feature the imprints of children’s teeth.
From the DNA, researchers can start to piece together some of the ancient woman’s physical traits and make some inferences about the world she lived in. “We determined that she had this striking combination of dark skin, dark hair, and blue eyes,” Schroeder says. “It’s interesting because it’s the same combination of physical traits that apparently was very common in Mesolithic Europe . So all these other ancient [European] genomes that we know about, like La Braña in Spain , they all have this combination of physical traits that of course today in Europe is not so common. Indigenous Europeans have lighter skin color now but that was apparently not the case 5,000 to 10,000 years ago.”
An artist's illustration of what the Scandinavian person who chewed the ancient piece of gum may have looked like. (Tom Björklund)
The gum-chewers’ family ties may also help to map the movement of peoples as they settled Scandinavia.
“The fact that she was more closely related genetically to people from Belgium and Spain than to people from Sweden, which is just a few hundred kilometers farther north, tells us something about how southern Scandinavia was first populated,” Schroeder says. “And it looks like it was from the continent.” This interpretation would support studies suggesting that two different waves of people colonized Scandinavia after the ice sheets retreated 12,000 to 11,000 years ago, via a southern route and a northeastern route along today’s Norwegian coast.
The individual was part of a world that was constantly changing as groups migrated across the northern regions of Europe. “We may expect this process, especially at this late stage of the Mesolithic, to have been complex with different groups, from south, west or even east, moving at different times and sometimes intermingling while perhaps other times staying isolated,” Jan Storå, an osteoarchaeologist at Stockholm University, says via email.
Additional archaeological work has shown that the era was one of transition. Flaked stone tools and T-shaped antler axes gave way to polished flint artifacts, pottery and domesticated plants and animals. Whether the region’s turn to farming was a lifestyle change among local hunter-gatherers, or spurred by the arrival of farming migrants, remains a matter of debate.
“This is supposed to be a time when farming has already arrived, with changing lifestyles, but we find no trace of farmer ancestry in her genome, which is fairly easy to establish because it originated in the Near East. So even as late as 5,700 years ago, when other parts of Europe like Germany already had farming populations with this other type of ancestry present, she still looked like essentially western hunter-gatherers, like people looked in the thousands of years before then,” Schroeder says.
“The ‘lack’ of Neolithic farmer gene flow, at this date, is very interesting,” adds Storå, who wasn’t involved in the research. “The farming groups would probably have been present in the area, and they would have interacted with the hunter-gatherer groups.”
The era’s poor oral hygiene has helped add even more evidence to this line of investigation, as genetic bits of foodstuffs were also identifiable in the gum.
Presumably not long before discarding the gum, the woman feasted on hazel nuts and duck, which left their own DNA sequences behind. “The dietary evidence, the duck and the hazel nuts, would also support this idea that she was a hunter-gatherer and subsisted on wild resources,” Schroeder says, noting that the site is littered with physical remains which show reliance on wild resources like fish, rather than domesticated plants or animals.
“It looks like in these parts maybe you have pockets of hunter-gatherers still surviving, or living side-by-side with farmers for hundreds of years,” he says.
Scientists also found traces of the countless microbes that lived in the woman’s mouth. Ancient DNA samples always include microbial genes, but they are typically from the environment. The team compared the taxonomic composition of the well-preserved microbes to those found in modern human mouths and found them very similar.
Satisfied that genetic signatures of ancient oral microbes were preserved in the woman’s gum, the researchers investigated the specific species of bacteria and other microbes. Most were run-of-the-mill microflora like those still found in most human mouths. Others stood out, including bacterial evidence for gum disease and Streptococcus pneumoniae , which can cause pneumonia today and is responsible for a million or more infant deaths each year.
Epstein-Barr virus, which more than 90 percent of living humans carry, was also present in the woman’s mouth. Usually benign, the virus can be associated with serious diseases like infectious mononucleosis, Hodgkin’s lymphoma and multiple sclerosis. Ancient examples of such pathogens could help scientists reconstruct the origins of certain diseases and track their evolution over time, including what factors might conspire to make them more dangerous.
“What I really find interesting with this study is the microbial DNA,” Anders Götherström, a molecular archaeologist at Stockholm University, says in an email. “DNA from ancient pathogens holds great promise, and this type of mastics may be a much better source for such data than ancient bones or teeth.”
Natalija Kashuba , an archaeologist at Uppsala University in Sweden, and colleagues have also extracted human DNA from ancient birch gum, from several individuals at a 10,000-year-old site on Sweden’s west coast . “It’s really interesting that we can start working on this material, because there’s a lot of it scattered around Scandinavia from the Stone Age to the Iron Age,” she says, adding that gums may survive wherever birches were prevalent—including eastward toward Russia, where one wave of Scandinavian migration is thought to have originated .
The fact that the discarded artifact survived to reveal so much information about the past isn’t entirely due to luck, Kashuba says. “I think we have to thank the archaeologists who not only preserved these gums but suggested maybe we should try to process them,” she says. “If it hadn’t been for them, I’m not sure most geneticists would have bothered with this kind of material.”
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