No, we are not talking about the afterlife, but about what happens to a decomposing body just after cell death. Far from being “dead”, (unappealing an idea as it may be) a rotting corpse is in fact full of life.
In short, things are about to get morbid. Many scientists think of a dead body as an oddly shaped stone with a vast and complex ecosystem, which emerges shortly after death, as the decomposition process gradually evolves. This begins a few minutes after death, in a process called “autolysis” or self-digestion. Shortly after the heart stops beating, the cells find themselves deprived of oxygen and their acidity levels increase as toxic by-products from chemical reactions start to build up inside the body.
Enzymes start to digest the cell membranes and are “released” as the cells decompose. This generally starts in the liver – which is rich in enzymes, and the brain, which contains high levels of water. Finally, other tissues and organs start to decompose in this way. The damaged blood cells start to spread out, discolouring the skin. Body temperature drops, acclimatising to the environment, and then the body starts to become stiff. The stiffness spreads out, starting in the eyelashes, the jaw and the neck muscles, before finally reaching the limbs.
While we are alive, our muscle cells contract and relax via the action of two filamentous proteins (actin and myosin) which slide over one another. After death, the cells are cut off from their energy source; the muscles thus become completely rigid, and lock out the joints. During these early stages, the corpse’s ecosystem is composed mainly of bacteria that was already present when we were alive. Every part of the body in fact houses a specialised microbial community. Unsurprisingly, the largest of these communities resides in the intestines, which hold billions of bacteria, and possibly thousands of different species of bacteria.
The intestinal microbiome is one of the most widely studies subjects in biology at the moment. We now know that it plays a role in a plethora of conditions and diseases, in autism and depression, as well as irritable bowel syndrome and obesity. But researchers still know only very little about these microbial passengers, and less again about them after we die. A 2014 study led by legal medical specialist Gulnaz Javan of Alabama State University in Montgomery, suggests however that shortly after death, the immune system stops functioning, allowing bacteria to spread freely throughout the body. This usually starts in the intestines, at the point at which the large and small intestines meet.
According to researchers, these intestinal bacteria start to digest the intestines from the inside, using the same cocktail of chemicals used to evacuate damaged cells. They then invade the blood capillaries of the digestive system and the lymph nodes, spreading first throughout the liver and spleen, then to the heart and the brain. Javan’s study suggests a fairly precise “microbial clock”. He in fact showed that bacteria reach the liver around 20 hours after death, and that it takes them at least 58 hours to spread to all of the organs.
Thus putrefaction has started. This is associated with a marked change in the aerobic bacteria, which need oxygen to develop, which transform into anaerobic cells which can only develop in the absence of atmospheric air or oxygen. These feed on the body’s tissues, fermenting sugars to produce gaseous by-products such as methane, hydrogen sulphide and ammonia, which build up in the body and swell the abdomen. This leads to further discolouration of the body. As damaged blood cells continue to spread out from from the disintegrating blood vessels, the anaerobic bacteria convert heamoglobin molecules (which once transported oxygen around the body) into sulphemoglobin. The presence of this molecule in the blood gives the body a “marble” or greenish effect, which is characteristic of a body undergoing active decomposition.
When a decomposing body begins to drain, it becomes fully exposed to its environment. At this stage, the corpse’s ecosystem becomes “properly” established. Two species closely linked to decomposition are flies and bluebottles. Corpses release a sickly smell composed of a complex cocktail of volatile compounds which change as the process of decomposition progresses. The insects detect the odour using specialised sensors on their antennae, and land on the corpse to lay their eggs in the orifices and open wounds.
Each fly lays about 250 eggs which hatch within 24 hours, giving birth to small maggots. These feed on rotten flesh, and develop into larger larvae, which feed for several hours before transforming again. Fattened up, these larvae transform into flies, and the cycle repeats until there is nothing left to eat. The presence of flies attracts predators such as beetles, mites, ants, wasps and spiders, which feed on or infest the flies’ eggs and larvae. In the absence of scavengers, the larvae are responsible for the elimination of soft tissue.
When there is nothing left to eat, the larvae leave the body, and their trails can be observed after decomposition is complete, in the form of deep trenches in the soil surrounding the body. It is estimated that an average human body is made up of 50 to 75% water, and for every kilogram of dried body mass, 32 grams of nitrogen, 10 grams of phosphorous, 4 grams of potassium and 1 gram of magnesium are released into the soil. Firstly, the decomposed body kills part of the nearby vegetation, possibly because of the toxicity of the nitrogen or due to antibiotics found in the body that were secreted by the insect larvae while they were feeding on the flesh.
But at the end of the day, decomposition is beneficial for the surrounding ecosystem. The microbial biomass surrounding the corpse becomes richer than in other nearby areas. Roundworms, which are linked with decomposition and attracted by filter nutrients, become more numerous, and plant life thus becomes more diverse. Future research on how body decomposition alters the surrounding environment could provide a new way of locating murder victims whose bodies were buried in shallow ground. The analysis of soil could also provide an indication of their time of death.
(PS, bon appetite!)