Human decomposition is as macabre as it is natural and inevitable. A body breaking down after its system has ceased to function is as interesting and as raw of a function as watching a live birth, or watching one animal stalk and kill another. Knowing about the process and the amount of time elapsing during each step is vital for medical professionals who take organs and tissue from donors and determine causes of death and disease. Forensic investigators use their knowledge of the process to figure out when murders occurred, which may help put guilty people in jail and keep innocent people out. Since decomposition is something that will happen to most people, at least those who are not cremated, it is a process for which nobody should be chastised for taking an interest.
Death is defined as the moment that the heart stops beating. Right away blood will stop circulating and bringing oxygen to every edge of the body. The body begins to cool at a rate of about 1-1.5 degrees Farenheit per hour, and blood will pool inside the body creating discoloration and a filmy appearance to the eyes. This process of cooling is called algor mortis or more eerily the "death chill", which is why recent death can often be determined by a touch-test alone.
After about three hours, rigor mortis occurs. The cadaver stiffens due to depletion of ATP (adenosine triphosphate), a protein that keeps the organs and muscles elastic. This state is the reason people use the slang term "stiff" in detective novels when referring to a dead body, and may be one of the most familiar phases of the decomposition process. While skin may dry out at this point and begin to shrink and recede, it should be noted that, contrary to legend, fingernails and hair do not continue to grow after death.
After 36-48 hours, rigor mortis will subside and the body will become soft. In a body left unattended or that has not yet been embalmed, insects will begin to feed on the flesh, laying eggs which will hatch into larvae that also feed. These creatures will not be alone in consuming the body, however. In fact, in the following process of postmortem autolysis begins, wherein enzymes and bacteria in the body will begin to consume it from the inside out. These are organisms, bacteria mostly, that live in the body of a human being and assist in digestive processes-and after human death, they multiply rapidly. These bacteria create a build-up of gasses inside the body as they consume it, which leads to the type of bloat that exists in corpses in the aftermath of genocide such as the one that occurred in Rwanda in 1994.
When enough pressure builds up inside the body the intestines will rupture and the putrefaction phase will begin. The multiplication of bacteria eventually liquifies the different organs of the body as the cell bodies separate from each other. Within a year or two, all flesh is completely broken down; however, to the delight of anthropologists everywhere, the skeleton can remain intact for hundreds of years.