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.