Scattered across Greenland's coastline are heaps of ancient garbage, bones, shells, animal waste, and broken objects, frozen solid for thousands of years. The objects inside have told archaeologists a great deal about who lived there. It turns out the bacteria have stories too.
A study published in Frontiers in Microbiology analyzed the bacteria preserved inside ancient garbage heaps, known as middens, at sites spanning 4,500 years of human habitation in Greenland. The researchers found bacterial signatures from human and animal activity still detectable centuries after the people who left them had gone, including bacteria linked to food poisoning, botulism, and toxic shock syndrome. As Arctic temperatures rise and permafrost thaws, the question of what happens to those bacteria is becoming harder to ignore.
“These middens in the cold Arctic acted like long-term natural experiments. Human- and animal-associated bacterial signals, including opportunistic bacteria and bacteria carrying antibiotic resistance genes, have remained detectable in them many centuries later as the legacy of human activity: for example, livestock farming by the ancient Norse,” said co-author Frank Møller Aarestrup in a press release.
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Ancient Greenland Middens Preserve Centuries of Bacterial DNA
Between 2020 and 2021, Aarestrup and colleagues collected samples from middens across West and South Greenland, identified through the Greenland National Museum and Archives registry. The sites covered multiple waves of human settlement, from Paleo-Inuit cultures dating back to around 2,500 B.C.E., to Norse descendants of Vikings who lived there between the 10th and 15th centuries, to early modern Danish settlers arriving in 1721.
At Norse sites, the team also collected soil samples from historic winter enclosures and summer grazing grounds, looking for traces of the animals those communities kept. They used DNA sequencing to reconstruct bacterial communities from the samples and compared their findings to soil taken from permafrost areas with no history of human settlement.
The middens contained between 9 and 202 bacterial species each, for a total of 1,207 species across all sites. Many had never been described before and could only be placed in broad categories.
Bacteria Trace Seals, Livestock, and Human Settlement
The bacterial communities found in the middens reflected the human and animal activity that had produced them. Middens from an early colonial-era settlement in Nuuk, which contained decomposing seal skins, were rich in a bacterium that is a major cause of food poisoning. Middens filled with animal carcasses contain bacteria that live in the gut of many animals. Early Norse middens with decomposing bones contained unknown species not found elsewhere.
The Paleo-Inuit middens showed the most soil-like bacterial communities of all the sites, suggesting that the microbial imprint left by humans and animals fades over time but does not disappear entirely.
The researchers also found a wide variety of antibiotic resistance genes preserved in bacterial genomes, with some of the same genes appearing in both ancient and contemporary soil layers, suggesting that antibiotic-resistant microbes have persisted in permafrost for centuries.
Why Thawing Middens Pose Little Risk for Now
Once released into runoff water as permafrost thaws, the ancient microbial communities appear to be replaced by local contemporary bacteria, meaning the pathogens are not spreading far from the middens themselves.
“However, it is not known whether the risk of release of pathogens will increase with increasing temperatures, or whether this might be greater in other Arctic regions,” said last author Anders Priemé.
The Arctic is currently warming three to four times faster than the global average. The middens have preserved a detailed biological record of 4,500 years of human life in Greenland, but how much longer that record will remain intact is an open question.
Read More: Global Thaw 10,000 Years Ago May Have Fueled Volcanoes and Sped Up Continental Drift
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- This article references information from a study published in Frontiers in Microbiology: Microbial composition of archaeological middens: Tracing Human Footprints Through Centuries in Greenland's Ancient Settlements