The elderly woman was sprawled on her back in the dirt, head resting to one side, elbows bent as though she was about to prop herself up. Dead three months now, her face was no longer recognizable. Her skin had thinned to a gossamer shroud over bone. She was among more than 150 corpses scattered beneath the trees, rotting in the open air or covered in plastic, on roughly three wooded acres.
To an outsider, the scene might look like a serial killer’s dumping ground, but it was just another day at the University of Tennessee, Knoxville’s Anthropology Research Facility, popularly known as the “body farm,” the first of only a handful of such facilities in the world where researchers study the science of human decay and law enforcement officers train to recover human remains at crime scenes.
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The dead woman was there to play her part in a developing frontier in forensic crime solving: analyzing and interrogating the suite of trillions of microorganisms and other creatures that are witness to our deaths.
“It’s an exciting time,” said Dawnie Steadman, director of the school’s Forensic Anthropology Center — through which the body farm operates — standing in the shade to escape the nearly 95-degree heat one morning in late May. “We’re in an age of technology where the microbes can help provide new answers about time of death, but also whether a body was moved, and medical conditions inside the body that can help identify a person.”
Calculating time since death, also known as the post-mortem interval, is an important aspect of forensic investigation, and is one of the focuses of body farm research. When an individual is unidentified, the post-mortem interval can help investigators narrow down who they might be based on missing persons records. “If we say, well, this individual passed away at least a year ago,” Steadman said, “then we know not to look at recent cases.”
That can help to shrink a pool of thousands of missing-person cases, she explained. More than 600,000 people go missing in the U.S. each year, according to the National Missing and Unidentified Persons System, and 4,400 unidentified bodies are recovered annually. As many as 1,000 of those bodies remain unidentified for more than a year.
Another reason establishing time since death is important is that it helps crime investigators evaluate alibis of potential perpetrators in homicide cases. Of the more than 16,000 murders in the U.S. in 2018, nearly 40 percent of them went unsolved. “If somebody has an alibi for six weeks ago, and we think it’s more like two to four weeks ago when this victim died, then that suspect may go back in the suspect pool,” Steadman said.
However, determining just when someone died is difficult. In the early hours and days after death, medical examiners rely on three hallmark measurements: algor mortis (body temperature), rigor mortis (stiffness), and livor mortis (the settling of the blood). But those signs fade quickly.
As decomposition gets rolling, forensic anthropologists mark five physical stages of decay: “fresh,” during which a person still looks relatively normal; “bloat,” when the body fills with gases; “active decay,” when a corpse’s soft tissue decomposes; “advanced decay,” and finally, “dry skeletal remains.”
During each stage, experts pay close attention to maggots, the wormlike larvae of blowflies, wriggling in a corpse’s flesh. On a warm, clear day, it can take mere minutes for flies to whiff the slightest decay — like a flashing neon sign announcing a good place to dine and procreate. Their arrival marks the start of a biological clock that allows investigators to use the maggots’ life stages to approximate the time when the flies first colonized the body. But the technique, popularly portrayed on TV dramas like “CSI” and “Law & Order,” isn’t perfect. For example, a murderer who tucks his victim in a fridge, or wraps him in plastic for a few days, postpones fly colonization, thereby artificially shortening estimates of the post-mortem interval. Even rain delays the arrival of the insects. Contrary to TV depictions, flies are not foolproof.
That’s why crime solvers, forensic anthropologists, and other scientists are excited about the microbes in the necrobiome, the term often used to describe the entire ecosystem of life involved with decay, from large scavenging mammals to organisms invisible to the naked eye.
“Microbes are ubiquitous,” said Jennifer DeBruyn, a University of Tennessee soil scientist, as she squatted near the elderly woman’s body to investigate some fungus growing on her arm. “They’re present in summer, winter, indoors, outdoors, even when a body is sealed in plastic. We don’t have to wait for them to show up, like insects.”
Advances in DNA sequencing and machine learning are making it possible to identify the bacteria, fungi, and other microbes associated with decay and search for predictable patterns that may eventually provide a method for more precisely determining time since death. “Microbes are the main drivers of decomposition,” DeBruyn said. “For that reason, they hold a lot of potential for understanding the timing or the circumstances around found remains.”
The Forensic Anthropology Center was the brainchild of William M. Bass, a renowned osteologist, or bone specialist, who joined the forensic anthropology faculty at the University of Tennessee in 1971. In that position and in his previous job at University of Kansas at Lawrence, Bass often helped law enforcement identify victims’ remains. But there was a big difference between Kansas and Tennessee. In Kansas’ dry climate, police often brought him boxes of bones and scraps of mummified tissue. In humid Tennessee, the corpses arrived fresher, smellier, and teeming with maggots. Bass wanted to learn more about approximating time of death in such conditions, so he went to the dean and told him that he needed some land to put dead bodies on.
The dean said Bass should talk to the man in charge of the agricultural campus. Soon Bass and his students were setting up shop in a pig barn, where they studied unclaimed bodies provided by state medical examiners. In the beginning, they wanted to know the answers to basic questions, like how long it would take for a skull to become visible.
In 1980, Bass convinced the school to give him some land closer to campus behind the university medical center, where the hospital had burned trash for many years. He poured a 16-square-foot concrete slab and enclosed it with chain-link fencing. This is where he and his students could continue their studies, meticulously recording the patterns and timing of decomposition. Gradually the research expanded to record the arrival of blowflies, developmental stages of maggots, and other variables.
Some bodies were placed naked, others were dressed; a few were buried or covered in plastic, while some lay in the open air. Several bodies were even stowed in the trunks of vehicles or submerged in water to mimic crime scenes.
The center’s donor program was established in 1981, and since then roughly 1,700 people have gifted their remains to the University of Tennessee Anthropology Research Facility, now spanning roughly three wooded acres. A building dedicated to Bass houses the nation’s largest collection of contemporary skeletons, a classroom, a laboratory, and an intake area where donor bodies are received and processed. (At least 4,000 people have registered as pre-donors.)
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The donors, mostly ordinary people motivated by helping science and criminal justice, typically register and carry a card in their wallets indicating their intention to go to the farm. The facility offers free pickup and delivery from funeral homes within a hundred miles of Knoxville; beyond that area, families must arrange transportation. When donors arrive, they are offloaded into a garage area, where they’re weighed and measured. Scars, injuries, and tattoos are photographed. Hair, blood, and nail samples are collected. The corpses are either stored in a large refrigerator for 12 to 24 hours, or moved directly to the facility’s wooded acreage, where they remain until they’re fully skeletonized.
“We can see the daily (even hourly) changes to hundreds of donors over the years in different seasons, in different scenarios, and in different microenvironments within the facility,” Steadman said. “This gives us a great deal of data to help evaluate the time since death of specific cases.”
Larry Sennett is a retired police officer from Lexington, Kentucky, who now works as a supervisor for the state’s Department of Criminal Justice Training (DOCJT). He said the body farm is an “unparalleled” resource for educating officers on how to deal with found human remains, from carefully marking the perimeter of a gravesite to meticulously removing layers of soil to expose the skeleton and any related evidence, including bullets and small bone fragments. “They use the training in every death scene they process,” Sennett said. “Most officers across the world aren’t able to get [that] type of training.”
Christina Priddy, a detective with the Hardin County Sheriff’s Office, who is a graduate of DOCJT’s Kentucky Criminalistics Academy, said she worked on a recent case involving a couple who strangled and beat a man to death, then buried the body not far from their house. “Our timeline would suggest he was buried maybe a day or two,” she said. “We found the grave plot. I was able to give instruction about how to unearth his body without destroying evidence.” Both partners were convicted of murder.
Prior to the creation of the Tennessee body farm, data on post-mortem intervals came from studies on animal analogs, mainly pigs. The opportunity to study human remains was a game changer for forensic anthropology, which has rapidly expanded in recent years.
Western Carolina University’s Forensic Osteology Research Station, founded in 2007, was the second body farm facility. In addition to serving as a laboratory for studying human decay at an altitude of 2,271 feet in the Blue Ridge Mountains, the center has provided a training ground for cadaver dogs that can detect human remains.
The third and largest forensic anthropology research facility opened in 2008 at Texas State University, San Marcos. Spread over 26 acres, the facility has yielded valuable data about human decomposition in the hot Texas Hill Country and produced research on the scavenging rates of vultures. A second facility in Texas is located at Sam Houston State University, renowned for its criminal justice program, in the southeastern area of the state. Additional body farms have been created in Illinois, Colorado, South Florida, and northern Michigan, providing the opportunity to compare how bodies decompose in many different environments, from sub-tropical swamplands to arid deserts to snow-covered plains.
In 2016, Australia opened the first body farm outside the U.S., the 12-acre Australian Facility for Taphonomic Experimental Research on the outskirts of Sydney. Scientists there have found that bodies decomposing in the bush tend to undergo some degree of natural mummification, producing dry and leathery skin that stays preserved longer. A year later, a teaching hospital in Amsterdam received approval to study the decomposition of bodies buried in shallow graves. And this summer, a new body farm opened in Quebec, providing the opportunity for scientists to study human decay in a northern climate where winter temperatures can dip as low as -30 degrees Fahrenheit. Plans are in the works for body farms in other parts of the world as well, including the United Kingdom.
“We welcome more facilities because although some of our research is translatable to any environment, some questions are environment specific,” said Steadman. “For instance, we don’t know how bodies behave in permafrost, or if they’re covered with snow eight months out of the year. We can take guesses, but that would be the type of thing we could learn from a body farm in that environment. We can ask specific questions in each unique environment — we can also get a better sense of what’s universal.”
She is particularly excited about recent innovations that are helping to expand the research. “There’s only so much you can learn by looking at a body,” she said. “With new technology we can analyze things beyond the big three — temperature, humidity, insects — and look at chemistry, cell structure, proteomics, and more nuanced questions.
“We’re still looking at the same question — how long is this person dead,’’ she continued. “But technology allows us to dive deeper.”
Microbes kick into action as soon as we take our last breath. (Even a stint in cold storage won’t halt some microbes associated with decay — some organisms are capable of working at very slow rates in subzero temperatures.) When the heart stops pumping, the body’s immune system shuts down, and microorganisms in the gut start multiplying, rapidly consuming nutrients. The feeding frenzy, essentially consuming the body from the inside out, creates gases that make the body bloat.
Eventually, the pressure causes skin to rupture and fluids are released, nurturing different types of microbes, and inviting bacteria, fungi, and nematodes from outside. As liquids and nutrients leave the body, flesh begins to sag and grow brittle, exposing bone. In an outdoor environment, scavenging animals often finish the cleanup, stripping the bones bare.
Jessica Metcalf, a microbial ecologist at Colorado State University, has spent several years plotting the macabre ebb and flow of microbes in hopes of developing a new forensic tool. She calls it a “microbial clock,” and it’s made from groupings of species that rise and fall in complex, yet predictable ways over time. “As different nutrients become available, different microbes flourish, so you see different profiles at different time periods,” she explained. “An investigator can collect the microbes, and we can match them to a model based on experiments.”
In a 2016 study published in the journal Science, Metcalf and her colleagues mapped microbial activity during decomposition by examining both mouse cadavers in the lab and human corpses at the Sam Houston State University body farm. They found a consistent succession of microbes transforming proteins and lipids into such smelly compounds as cadaverine, putrescine, and ammonia in different seasons, varied soils, and even in separate species — mice and humans. In that study, the scientists reported accurately identifying time since death within a two- to three-day period during the first two weeks of decay.
Metcalf said her group’s most recent research shows microbes are predictable enough that even after 25 days of decomposition, investigators tracking skin and soil microbes can estimate time since death to within two to four days. “We’re using machine learning, like what Netflix uses to guess what we want to watch next,” Metcalf said. “We collect microbes at all these different time points, sequence the DNA, then compare an unknown sample to try to match what time point that set of microbes appears.”
Currently, Metcalf is collaborating with researchers at body farms in Texas, Colorado, and Tennessee to determine whether there’s enough consistency in the succession of microbes that are active in decaying corpses to develop a universal clock. “We’re still trying to understand how robust our model is, and our error rate, while testing for certain variables, including season, temperature, and geography,” she explained. “Is the succession of microbes general enough wherever the body is found? Or do we need a clock for each region?”
She thinks the research is on track to provide a forensic tool that could be used in death investigations within the next three to five years. However, some of her colleagues say it will take longer — possibly 7 to 10 years — for the research to meet the standards needed to be admissible in court.
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Another initiative probing the microbial witnesses to our deaths is informally known as the Human Postmortem Microbiome project, funded in part with a $843,000 grant from the National Institute of Justice. Jennifer Pechal, an entomology expert at Michigan State University, is one of several researchers at different institutions working on the project, which sprung from a meeting she had with Wayne County medical examiner Carl Schmidt at the 2014 American Academy of Forensic Sciences Conference.
“I was giving a talk and mentioned that I was looking for collaborations,” Pechal said. “He was in the audience because he needed some continuing education credits for his job. He came up to me later and said, ‘I’m just down the road — if you need bodies, we’ve got bodies.’”
To date, Schmidt’s office has swabbed microbes from the ears, noses, mouths, and rectums of nearly 3,000 arrivals — heart attack victims, drug overdoses, suicides, homeless people who die from hypothermia — at the Wayne County Medical Examiner’s Office, which serves the Detroit area. Similar to the body farm studies, by swabbing the corpses’ various parts, the scientists are able to identify the microorganisms that are present within the early hours and days after death. However, the dataset of corpses in Detroit is quite different from the body farms, where donors tend to be white and middle- to upper-class. The Detroit study represents an urban, industrial Midwest population.
“It’s a cross-sectional survey of how the microbial communities really are changing with a population of people who have not self-selected to donate their bodies to science,” Pechal explained. “They’re representative of an everyday person.”
So far, analysis of the data indicates that certain microbial patterns can help identify a victim’s sex, while other signatures may help narrow down the pool of missing individuals in cases where other identifying marks — like tattoos — are not available.
Another potentially helpful finding: Victims of drug overdoses appear to host distinct communities of microbes compared with cadavers of people who died of natural causes.
Body farm researchers are also considering how disease and the pharmaceuticals we take for everything from diabetes and cancer to hypertension and depression might influence the necrobiome and decomposition. Scientists at the Knoxville body farm are researching whether the bodies of people who suffered from diabetes are more attractive to insects, and looking into whether certain drugs may hasten or slow decomposition.
“We know cocaine speeds up maggots,” Steadman said, while barbiturates — according to case studies in the existing literature — appear to do the opposite. She’s optimistic that DeBruyn and her graduate students will find some new clues about these and other questions hidden in the soil.
One humid day in May, DeBruyn and four other women dressed in white Tyvek jumpsuits pulled three 200-pound men out of a giant freezer and placed them on orange stretchers similar to what medics use to carry patients over uneven terrain. The scientists sweated inside their suits in the 80-degree heat as they picked their way down a steep, wooded slope to a patch of virgin soil outfitted with special equipment that would monitor the temperature, moisture, and salts beneath the corpses. When they finally arrived at the site, the scientists heaved the men into place and plugged in the probes.
Over the next days, weeks, and months, they returned regularly to roll the corpses so they could take oxygen measurements and soil samples using stainless steel metal probes. Samples of the body fluids leaking into the soil were drawn with a syringe. Some of the soil and fluid samples were flash frozen in liquid nitrogen to preserve DNA and other biological material for sequencing at the lab, along with an assortment of other tests.
As they collected the samples and processed them in the laboratory, they could see two of the corpses were decomposing faster than the third, but they couldn’t explain why. It’s a common conundrum — the rate of decay frequently varies from corpse to corpse, even when steps are taken to maintain constant variables, like body weight and location.
“There’s an intrinsic variability in decomposition rates that I don’t think we have a good handle on yet,” DeBruyn said. “Maybe these people have different microflora, different diets, different medications. This makes finding a universal signature a big challenge.”
That’s why DeBruyn is among the scientists who are cautiously optimistic about the power of the microbiome to help solve crimes. “It has a lot of promise, but I think we’re a ways off from using it as a forensic tool,” she said. “We need to step back and observe the whole system — chemistry, maggots, microbes, soil. This is classic ecology, observing the ecosystem. But that’s not the forensic way; the forensic way tends to be looking at one specific thing.”
DeBruyn uses a kitchen analogy to explain why the nuances are significant. Current DNA sequencing of microbes associated with death basically asks for the names of the chefs in the kitchen, she said. But it may be more important to find out what types of cuisines they’re cooking, or what types of foods they’re using, or their style of cooking.
“There’s a lot of questions we can ask about the microbial communities in the same way,” she said, still squatting beneath the trees, examining the elderly woman’s fungus. “In the case of decomposing bodies, what tissues and molecules are they breaking down, and what sort of products are they putting out? These questions might be more useful for understanding the system.”
Standing up and stepping carefully through the leaf litter so as not to disturb the woman’s remains, DeBruyn joined Steadman on the path, where a group of students was gathering for a lesson in forensic evidence-collecting techniques. “I think at the end of these studies, we’ll have a lot more questions,” said Steadman, taking off her rubber gloves and walking through a pair of gates concealing the entrance to the facility. “And that’s a good thing — that’s the way science works.”
Rene Ebersole is a journalism professor at New York University’s Science, Health, and Environmental Reporting Program who writes about science and the environment. Her work has appeared in National Geographic, Audubon, Outside, Popular Science, The Nation, and The Washington Post, among other publications.
UPDATE: A previous version of this piece incorrectly described Jessica Metcalf as a microbial ecologist at the University of Colorado-Boulder. She is at Colorado State University.
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