First of all let’s look at what happens to the human body at the time of
death and soon after. At the very moment of death the heart stops
beating and the lungs stop breathing. This means that the cells in the
body will no longer receive blood and oxygen. Since the blood is no
longer being pumped through the body it will drain from the blood vessels
at the top of the body and collect in the blood vessels on the lower part
of the body. The upper part of the body will become pale and the lower
part of the body will become dark. If the person is lying on their back,
the front of their body and face will be very pale or even grey while
their back will be much darker and look almost like it is bruised. This
is called lividity or liver mortis and is one of the first things that a
scientist will look at to try to determine when someone died and if they
were moved after death.

At this point most of the cells in the body are still not dead. While
the brain cells die in the first few minutes after the heart stops,
muscle cells can live for several hours and skin and bone cells can stay
alive for days! How is this possible? Well, the cells use a different
type of respiration than when the heart and lungs were working. While
the person was alive the cells used aerobic respiration (with oxygen),
but after death the cells continue to survive using what is called
anaerobic respiration (without oxygen). However, one of the by-products
of anaerobic respiration is lactic acid. Lactic acid eventually builds
up and causes the muscles to stiffen. This is the same thing that
happens to a person’s legs when they run a long distance. The heart and
lungs can’t keep up with the demand so the leg muscles start to use
anaerobic respiration. In a living person this lactic acid will
eventually be cleared out by the circulatory system, but in a dead body
this isn’t possible so the entire body stiffens. This is called rigor
mortis. Rigor mortis usually starts about 3 hours after death and lasts
36 hours. Eventually all of the cells die and the body can no longer
fight of bacteria. The cells’ own enzymes and the enzymes of bacteria
begin to cause the body to decompose and the muscles lose their
stiffness. Like liver mortis, rigor mortis is another tool that
scientists can use to determine the time of death.

OK. Now comes the actual process of decomposition, or breakdown and
decay of the body. Decomposition can be broken down into 5 steps:

1. Initial decay
2. Putrefaction
3. Black putrefaction
4. Butyric putrefaction
5. Dry decay

Let’s look at each of these steps in more detail.

Step 1: Initial decay
Initial decay occurs from 0 to 3 days after death. Although the body
appears fresh from the outside, many things are going on inside the body
to contribute to the process of decomposition. The bacteria that are
normally inside the intestines of a living person begin to feed on the
contents of the intestine and the intestine itself. Eventually these
bacteria break out into the body cavity and start to digest other
organs. Since the intestine is no longer intact, the body’s digestive
enzymes, which were kept safely inside the intestine and stomach, leak
out and spread through the body helping to break down more organs and
tissues. At the same time, enzymes inside individual cells leak out and
digest the cell and its connections with other cells.

Let’s not forget about the insects! From the moment of death flies are
attracted to the smell of the decomposing body. Without the normal
defenses of a living body, these flies are able to lay their eggs around
wounds and other body openings (mouth, nose, eyes. etc.). Within 24
hours most of these eggs hatch and the larvae, or maggots, move into the
body to feed on the dead tissue.

Step 2: Putrefaction

Putrefaction occurs from 4 to 10 days after death. As the bacteria are
breaking down the tissues and cells they are also producing a lot of
gas. These gases include hydrogen sulfide, methane, cadaverine, and
putrescine. All of these gases really stink, but insects love the
smell. More and more flies start to show up along with beetles and
mites. The gases also cause the body to inflate forcing more fluids out
of the cells and blood vessels and into the body cavity. This provides
even more food for the bacteria and a nice warm living space for the
maggots.

Step 3: Black putrefaction

This stage occurs from 10 to 20 days after death. The bloated body
eventually collapses and the flesh has gotten creamy (like cottage
cheese). The exposed parts of the body have turned black and the body
really begins to stink. A lot of the fluids have now leaked out of the
body into the soil attracting more insects and mites. These insects and
mites will eventually consume most of the flesh on the body. Bacteria
are still at work also, and will consume the flesh if there are no
insects around. The temperature of the body also increases due to all of
the insect activity.

Step 4: Butyric fermentation

Butyric fermentation occurs from 20 to 50 days after death. All of the
remaining flesh on the body is removed during this time and the body
starts to dry out. It has a “cheesy” smell caused by butyric acid. This
smell attracts a bunch of new organisms to the body. Mold starts to grow
on the part of the body that is touching the ground and a lot of beetles
show up. Since the body is beginning to dry out maggots are no longer
able to eat the tough flesh. Beetles, however, are able to chew through
this tough material such as skin and ligaments.

Step 5: Dry decay

This stage occurs from 50 to 365 days after death. The body is now dry
and decays very slowly. Tineid moths and bacteria eventually eat the
person’s hair, leaving nothing but bones. As long as there are no large
animals around to carry them away, the bones can remain almost
indefinitely.

That brings us to the end of the decomposition process. Most of these
steps depend a lot on the climate. Temperature and moisture and the
presence of insects will affect how long this whole process takes.
Decomposition will occur much faster in the summer than in the winter and
also will take longer in a body that is buried than a body that is left
exposed on the ground.

References:
• Anderson, G.S. 2000. Minimum and maximum development rates of
some forensically important Calliphoridae (Diptera). Journal of Forensic
Sciences. 45: 824-832.
• Bornemissza, G.F. 1957. An analysis of arthropod succession in
carrion and the effect of its decomposition on the soil fauna. Australian
Journal of Zoology. 5: 1-12.
• Fuller, M.E. 1934. The insect inhabitants of carrion: a study in
animal ecology. Council for Scientific and Industrial Research. Bulletin
No. 82. 63 pp.
• Kamal, A.S. 1958. Comparative study of thirteen species of
sarcosaprophagous Calliphoridae and Sarcophagidae (Diptera) I. Bionomics.
Annals of the Entomological Society of America. 51: 261-270.
• Morovic-Budak, A. 1965. Experiences in the process of
putrefaction in corpses buried in earth. Medicine, Science and the Law 5:
40-43.
• Rodriguez, WC. and Bass, WM. (1985). Decomposition of buried
bodies and methods that may aid in their location. Journal of Forensic
Sciences 30: 836-852.
• Spennemann, D.H.R and Franke, B. 1995. Decomposition of buried
human bodies and associated death scene materials on coral atolls in the
tropical Pacific. Journal of Forensic Science. 40: 356-367.