Human decomposition is the breakdown of organic matter that begins once life ceases. This complex process involves a series of cascading physical and biological changes that unfold over time. The timeline is highly variable, depending on a multitude of internal and external factors. Decomposition starts immediately, but the visible and dramatic changes occur along a progression of stages, which can take anywhere from days to several years to fully complete.
The Initial Changes Immediately Following Death
The first physical changes begin immediately upon the cessation of circulation and respiration. One of the earliest indicators is the cooling of the body, known as algor mortis. The internal temperature drops until it matches the ambient environment, typically cooling at a rate of approximately one to one and a half degrees per hour after the first hour.
Another early sign is livor mortis, or lividity, which results from the settling of blood within the capillaries due to gravity. This pooling causes a reddish-purple discoloration on the parts of the body closest to the ground. Livor mortis generally begins to appear between two and four hours after death and becomes fixed—meaning it will not blanch when pressed—after eight to twelve hours.
The muscles then begin to stiffen in a process called rigor mortis, caused by the depletion of adenosine triphosphate (ATP), the energy molecule necessary for muscle relaxation. This stiffness starts in the face within two hours and progresses to the limbs, typically completing throughout the body between six and eight hours after death. Rigor mortis generally remains for about twelve hours and then disappears as muscle proteins break down, usually gone by thirty-six hours.
The Biological Stages of Decomposition
The true internal breakdown begins with autolysis, or self-digestion, which starts immediately after death. When blood flow and oxygen cease, cells become acidic, causing their walls to rupture and release digestive enzymes. These enzymes begin to break down the body’s tissues from the inside out, most rapidly affecting organs with high enzyme content, such as the liver.
The next major shift is the bloat stage, driven by internal bacteria residing in the gastrointestinal tract. These microbes, no longer constrained by the immune system, consume tissues and release gases like methane and hydrogen sulfide. The accumulation of these gases causes the abdomen and other body parts to noticeably swell, sometimes doubling the body’s size, typically starting within three to five days after death. The gases also produce strong odors and cause the skin to discolor, often turning a greenish-red.
Following the bloat stage is active decay, where built-up gases escape and the body begins to collapse. During this phase (five to eleven days postmortem), soft tissues, organs, and muscles start to liquefy. Fluids leak from the body, and the body loses the majority of its mass. The final stages, advanced decay and skeletalization, occur when the remaining soft tissue dries out, leaving behind only bones, cartilage, and dried skin. This transition can take anywhere from a few weeks to several years, depending on the surrounding conditions.
Key Variables That Determine the Rate of Decay
The rate at which decomposition progresses is heavily dependent on environmental conditions, with temperature being the most powerful influence. Warmer temperatures significantly accelerate the process because heat increases the activity of the bacteria and enzymes responsible for breaking down tissue. Conversely, cold environments drastically slow decay, as low temperatures inhibit microbial growth and can halt the process entirely if the body freezes.
The environment also dictates the timeline, as bodies in the open air typically decompose much faster than those submerged in water or buried. Water submersion generally slows decay because cooler temperatures inhibit bacterial growth, though aquatic life can complicate the speed. Burial also limits access for insects and air, which slows the rate, with decomposition being slowest in deep burial or low-oxygen environments.
Accessibility to insects and scavengers is another major factor that can drastically accelerate the timeline. Flies are often the first to arrive, laying eggs that hatch into larvae that consume soft tissue and can reduce a body to a skeleton in a matter of days under optimal conditions. Internal factors also play a role, such as a larger body mass with more fat tissue, which can decompose more rapidly.
The cause of death can also influence the rate; for example, an infection like sepsis can introduce a higher bacterial load, speeding up the initial breakdown. Clothing or other coverings can delay decomposition by limiting exposure to external elements and insect activity. The interplay of all these variables makes the duration of any decomposition stage highly context-specific, ranging from days to many years to reach the skeletal stage.