Forensics 101: Determining Time Since Death Based on Decomposition

In our continuing series on forensics and the skills specific to forensic anthropologists, we’re starting at the beginning when it comes to death investigation by discussing after-death processes.

It’s a sad but true fact that the body breaks down following death in ways that aren’t pretty. Many of us have gotten a glimpse of this process from those unrecognizable lumps commonly labelled 'road-kill'. But whether human or animal, we all undergo the same processes post-mortem. In the case of a murder or accident investigation, forensic scientists can use this progression to determine when the victim died.

Microscopic changes start to occur at the cellular level immediately following death, while visible changes become evident within a few hours:

  • blood no longer circulates and gravity causes it to pool in the lowest parts of the body, turning that flesh a dark purple-red (pallor and livor mortis)
  • body temperature drops at a rate of 1 – 2 oC per hour (algor mortis)
  • the body starts to stiffen approximately 2 to 6 hours following death, persisting for 24 to 84 hours (rigor mortis).

Following these changes, the process of putrefaction begins.

When human remains are actively in this putrefaction phase, a forensic anthropologist may be called in as a consultant to the medical examiner. The general rule is that the more decomposed the remains, the greater for the need for a forensic anthropologist’s expertise.

During putrefaction, the bacteria naturally found in the intestinal and respiratory tracts move into the body’s soft tissues, breaking down the component materials ― proteins, fats and carbohydrates ― and producing the noxious gases associated with the bloat stage of decomposition ― hydrogen sulfide, putrescine, cadaverine, methane, acetone and numerous alcohols and acids. Tissues in the body then liquefy in a specific order ― intestinal tract and circulatory system, lungs and related tissues, urinary tract organs, brain and nervous tissue, skeletal muscles and, finally, connective tissue, leading to complete skeletonization.

It’s a very predictable progression, but one that wasn’t well studied in the past because of society's ethical and spiritual conventions regarding death. However, since 1971, the Forensic Anthropology Center at the University of Tennessee has specialized in the controlled study of decomposed human remains under the leadership of Dr. William M. Bass. Based on the research done at this facility, forensic science has leapt forward, and much of this knowledge has been invaluable in crime scene investigations. Detailed information is now available about those variables that significantly affect human decomposition rates ― temperature, insect colonization, burial, carnivore scavenging, clothing, body weight, trauma and soil pH.

Armed with knowledge of the decomposition process and the variables that can affect it, forensic scientists can estimate with significant accuracy a ‘time since death’ window for a set of fleshed human remains at a crime scene. This is particularly important for highly degraded remains when victim identification is already problematic. As well, a reliable time since death window can contribute to information on how death might have occurred and who might have been involved.

In the next Forensics 101 post, we will be looking at sexing an unknown victim, based purely on skeletal markers. We hope to see you there…

Photo credit: Wikimedia Commons