A body falls in the woods and although no one is around to hear it, a clock starts ticking. It’s not made of gears or springs, but of bacteria, fungi, and other microbes.
The corpse dumps a huge flood of nutrients into the earth—a blend of fats and proteins that stands out among the carbohydrates typically found in leaf litter. Quickly, a dedicated coterie of bacteria, fungi, and nematode worms emerges to dine on this artisanal feast. “There’s this complete overhaul of the living community in the surrounding area,” says Jessica Metcalf from the University of Colorado, Boulder.
Now, Metcalf’s team, led by Rob Knight at the University of California, San Diego, have shown that these cadaver microbes—let’s call it the necrobiome—change in a predictable clock-like way. It’s consistent in the species that show up, the order in which they arrive, and they pace at which they do so, seemingly regardless of soil types, seasons, or even species—the mouse necrobiome is similar to the human one.
This means that forensic investigators should be able to tell how long ago someone died, to within two to four days, by swabbing and sequencing the microbes that pervade and surround their body. That’s comparable to other indicators of time of death, such as the chemical content of the surrounding soil, or the developmental stages of corpse-eating blowflies. But while insects are scarce in the winter, the necrobiome clock ticks throughout the year.
The necrobiome will likely be used as one of several sources of forensic evidence, all complementing each other. (The microbes inside a dead human body provide more clues; last year, other researchers showed that these microbes also change in clock-like way, post-mortem.) “When you have a case that’s being treated as a homicide, you want to collect as many different lines of physical evidence as possible,” says David Carter, a forensic scientist from Chaminade University of Honolulu, who was part of Metcalf’s team.
Back in 2013, Metcalf found predictable patterns in the necrobiomes of decaying mice, and could estimate the rodents’ times of death to within three days over a seven-week period. Building on those results, she left dead mice to rot on soil from three very different habitats—desert, prairie, and alpine forest—each with its own starter communities of microbes.
The team also worked with human cadavers at the Sam Houston State University Southeast Texas Applied Forensic Science (STAFS) Facility, where people will their bodies to forensic research. The STAFS staff left two bodies to decay on the facility’s grounds, two in the winter and two in the summer. “For this to be relevant for forensic science, we needed to show that this microbial clock really exists in an outdoor scenario, where you have scavengers, insects, and daily temperature fluctuations,” says Metcalf.
Despite the many variables in these experiments, the team found that both mouse and human corpses are quickly colonised by similar groups of bacteria, especially those that specialise in digesting fats and proteins. There are also plenty of microbes that process nitrogen, which is unsurprising given that corpses eventually rupture and leak nitrogen-rich fluids into their environment.
These colonizing bacteria are found ubiquitously in soils, but are usually rare. When animals die on top of them, it must be like manna from heaven—and their populations explode. “Dead animals are big, concentrated blobs of protein and fat, and these microbes are lurking in the soil ready to start eating our bodies when we die,” says Steven Allison from University of California, Irvine, who was not involved in the study.
This is a classic case of ecological succession, where living things colonize a new habitat in predictable waves. A scorched forest gets settled by mosses, ferns, shrubs, and eventually trees. Newborn babies get colonised by milk-digesting bacteria and then plant-busting ones. A whale carcass, sinking to the ocean floor, becomes host to hagfish, crabs, snails, and bone-eating snot-flower worms. A human corpse is no exception—in death, we become just another haven for life.
Our post-mortem fates are so foreseeable that Metcalf could use the data from mice buried in one kind of soil to estimate the time of death for animals buried in another. “We could even generate a clock based on the mouse experiments and predict the time of death of humans decomposing outdoors,” she says.
But “there’s still a lot of unexplained variation,” says Allison. “The predictable part of the corpse microbial communities is actually a fairly small fraction but since the communities are diverse, maybe that’s enough to make a good prediction of time of death in most cases.”
“It’s promising because with a relatively small number of experiments, we’re seeing predictable and reliable trends—but that’s a weakness too,” adds Carter. “We’ve only processed a small number of samples, so we need more data.” He wants to know if the necrobiome is predictable in other conditions. What about the tropics? What if there’s gunshot residue on somebody’s skin? What if they’re on antibiotics? Or they’re an alcoholic? “There are a lot of variables that it would be nice to investigate,” he says. “But that’s probably beyond my lifetime.”