When a forest is scorched by a fire, you can safely bet that grasses will be the first to regrow, followed by small bushes, and eventually trees. When a newborn baby is born, it will first be colonized by bacteria that can digest milk, followed by those that break down plant fibers. When a dead whale sinks to the ocean floor, it will first be consumed by writhing hagfish and scavenging sharks, then crustaceans and snails, and eventually mats of microbes. These are all examples of ecological succession, where new habitats are colonized by orderly and predictable waves of species.
The same concept applies to human corpses. When we die, our bodies—essentially concentrated globs of protein and fat—are readily devoured by a thriving menagerie of bacteria, fungi, and other microbes. And according to work from Jessica Metcalf, from Colorado State University, these species—the necrobiome—roll up in predictable waves, just like those that arrive at burned forests, newborn infants, and sunken whales. What’s more, the necrobiome turns over with clocklike predictability. As I wrote in 2015:
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.
You can learn more about the necrobiome study in the video below—the latest in a series of online films produced by HHMI Tangled Bank Studios, which adapt the stories in my book, I Contain Multitudes.