Decomposition was assumed to be an environmental process. Temperature, humidity, soil fauna, insect access, UV exposure — these external variables determine how quickly organic matter breaks down. A body in the tropics decomposes differently from a body in the desert. Forensic textbooks organize decomposition by climate zone, by season, by exposure.
Metcalf and colleagues (Nature Microbiology, 2024) placed 36 cadavers at three forensic anthropological facilities across two distinct climate zones — temperate forest and semi-arid steppe. They sampled the microbial communities over time and found that the same core set of 20 bacteria and fungi appeared on every body. The same species, in the same succession order, on the same timeline. Climate didn't matter. Season didn't matter. Geography didn't matter.
The body carries its own destruction schedule.
Those 20 organisms are not ambient. They are not recruited from the local soil or blown in by regional winds. They are present on every human body and begin their program upon death. The succession is deterministic: early colonizers create chemical conditions that enable the next wave, which enables the next. The timeline is so consistent that machine-learning models trained on microbial community composition can estimate time of death — a “microbial clock” — with accuracy that improves on entomological methods, which depend heavily on temperature.
This inverts the standard decomposition model. The textbook says: the environment determines the rate and character of decay. The data says: the organism carries its own decay program, and the environment modulates it at the margins. The 20-microbe succession is the primary signal. Temperature and humidity are secondary modifiers. The difference between a body in Texas and a body in Colorado is not which microbes appear but how fast the same program runs.
The general principle: when a process appears environment-dependent at coarse resolution, it may be organism-determined at fine resolution. The environmental variables are real but they operate on a fixed substrate — a program that the body itself provides. Decomposition is not something that happens to a body. It is something the body does, using residents it carried in life, on a schedule encoded in the microbial community's ecology. The environment sets the tempo. The body wrote the score.