A burial excavation during a law enforcement training in June 2019 at the Anthropology Research Facility (also known as the Body Farm). Credit: Steven Bridges/University of Tennessee
The University of Tennessee Anthropological Research Facility, colloquially known as the Body Farm, has been around since 1972. While the facility held up to about 20 bodies in the early years, it expanded to 150 or more in 2007.
That’s a lot of bodies—and multiple researchers began noticing that bodies donated and placed in the soil at the same time were not decomposing in the same way.
“The bodies experienced identical environmental conditions, but we saw big differences in how quickly they decomposed,” said microbial ecologist Jennifer DeBruyn, at the University of Tennessee’s Anthropology Research Facility.
Generally, changes in decomposition are attributed to environmental factors like temperature or humidity, which is why Northern Michigan University’s Body Farm was so important when it opened in 2017 as the world’s first cold-weather body farm. But, as DeBruyn said, the environmental conditions were identical in these situations, suggesting the culprit was something inside the body, rather than out.
For their study, published in mSphere, DeBruyn and her research team studied 19 bodies that had been donated and placed in the outdoor facility between February 2019 and March 2020. Temperature and humidity data were recorded hourly by remote tags, and the team collected soil samples at regular intervals throughout decomposition. They also analyzed the bacterial and fungal composition of fluids produced by the bodies, as well as the composition of the surrounding soil during “active decomposition,” which lasts until the body stops releasing fluids and the abdomen cavity collapses.
According to the study results, most of the bacterial communities in the fluid released into the soil belonged to the Firmicutes and Proteobacteria phyla, which include dominant species typically found in the human gut. The majority of the fungal communities in the fluids were associated with the class Saccharomycetes.
That data still didn’t explain the observed variation in decomposition, however. Then, the researchers recorded more diversity in the soil microbe communities in the soil around the bodies than they’d observed in the decomposition fluids. That highlighted a possible connection—body mass index.
“We know from plant litter studies that even slight changes in tissue chemistry can change the microbial decomposers,” said DeBruyn. “More fat tissue—in a body with higher BMI—means more moisture and a higher ratio of carbon to nitrogen, relative to a body with a lower BMI.”
The BMIs of the 19 donated bodies the researchers studied ranged from 14.2 to 55.1. Examining the data, the team found that in the soil beneath and near individuals who were underweight (BMI less than 18.5) and normal weight (a BMI between 18.5 and 26), diversity decreased in bacterial communities. In obese and overweight individuals (with BMIs above 26), diversity remained mostly constant.
Understanding the mechanism behind this BMI observation will require more work, DeBruyn said. The same is true of other observations made during the study. For example, the researchers noticed that the soil around people who had been treated for cancer showed less microbial species richness, perhaps because the chemotherapeutic agents inhibited the growth of microbes.
Regardless, the study is the first to identify connections between intrinsic factors and soil microbial populations, and is a huge step toward the longer-term goal of understanding how differences in human bodies affect soil post-mortem.
“The biggest problem with these humans is that we’re just a big mixed bag of chemicals, and that’s why this kind of study really hasn’t been done before,” said DeBruyn. “Our paper is really one of the first to use a large enough sample size of donors to reveal these patterns.”