Edexcel A (SNAB) A Level Biology:复习笔记6.1.1 Determining the Time of Death

Determining the Time of Death

  • Forensics is the use of science in the investigation of criminal activities
    • It involves the collection and analysis of evidence from a potential crime scene e.g. the scene at which a dead body is found
  • Determining the time of death (TOD) of a body is a very important component of forensic science
    • The TOD can be used during a police investigation to provide information about the circumstances surrounding the death of a person
  • In order to accurately estimate TOD, there are several factors that must be established
    • Extent of decomposition
    • Stage of succession
    • Forensic entomology
    • Body temperature of the deceased
    • The degree of muscle contraction

Extent of decomposition

  • The process of decomposition begins soon after death
    • Decomposition is carried out by organisms known as decomposers e.g. bacteria and fungi
      • Enzymes secreted from the cells of these organisms break down biological molecules in dead tissue
  • The time since death can often be established visually by looking at the appearance of a body that is decomposing
    • Decomposers break down cells and tissues over the course of a few days
      • At this stage in decomposition the appearance of the skin can be a helpful indication of time since death; skin will often appear greenish in colour
    • The next stage of decomposition involves the breakdown of tissues and organs by micro-organisms over the course of a few days or weeks
      • This process produces gases, such as methane, which will lead to bloating
      • The skin will blister and fall off the rest of the body
    • A few weeks after death the remains of the soft tissues will turn to liquid which becomes visible as it leaves the body
    • This process will continue over the course of months or years until only a skeleton remain
    • After a few decades or centuries, the skeleton will disintegrate until nothing remains
  • The rate of decomposition will be affected by factors such as temperature and availability of oxygen
    • Decomposition would be slower in anaerobic conditions and at lower temperatures but would be faster at high temperatures

Stage of succession

  • Succession refers to the change in the types of organisms found in a habitat over time
    • This is often an ecology term that is applied to a habitat such as a pond or woodland, but in this case the habitat is the dead body
      • The difference between succession in ecology and in forensics is that in an ecosystem the early pioneer species are out-competed and disappear as the system matures, while in a dead body all of the newly arriving species remain as decomposition progresses
  • The stage of succession of a body can provide information about the estimated TOD
  • Above ground the body would undergo the following stages of succession
    • Bacteria will be found in and on the dead body immediately after TOD
    • As tissue decomposition sets in it creates ideal conditions for flies to lay eggs and their larvae to hatch
    • As more soft tissue is consumed by the fly larvae it creates favourable conditions for beetles to establish
    • When tissue dries out over time flies will leave the body as they prefer a moisture-rich environment
    • Beetles, however, can decompose dry tissue so they will remain on the body
    • Once all tissues have been decomposed most organisms will leave the body
  • These succession stages will differ depending on where the body is located as the accessibility to insects and availability of oxygen will be affected e.g.
    • Buried in soil
    • Buried in a coffin
    • Under water

Forensic entomology

  • A dead body provides an ideal habitat for many species of insects; the study of these insect colonies is known as forensic entomology
  • Different insect species will colonise a body at different times after death, providing information about the TOD
    • Flies will be found on the body within a few hours after death, while beetles will only colonise the body later
  • Another clue that insects can provide is the stage of life cycle they are at
    • E.g. blowfly eggs will hatch after about 24 hours so if larvae are present on the body it indicates that the person died more than 24 hours ago
      • Other insects have longer life cycles, so if only blowfly larvae are found it indicates that only 24 hours has passed since TOD
    • Factors that might affect the progression of insect life cycles include
      • Drugs that may be present in the body
      • Humidity of the surroundings
      • Oxygen availability
      • Temperature

Body temperature

  • Respiration and other metabolic processes produce heat in living organisms
    • This heat is necessary for maintaining our body temperature at around 37 °C during life
  • Once a person dies metabolic reactions will eventually come to an end
    • Since no more heat is produced the body temperature drops until it reaches the temperature of the surrounding environment
    • This process of cooling is known as algor mortis
  • Body temperature decreases by 1.5-2.0 °C per hour, therefore providing forensic scientists with a way to determine the TOD based on the temperature of the body
  • Certain conditions will affect the rate at which body heat is lost e.g.
    • Air temperature
    • Surface area : volume ratio
    • Presence of clothing

Degree of muscle contraction

  • Muscles in the body begin to contract about 4-6 hours after TOD, leading to a general stiffening of the body known as rigor mortis
  • Rigor mortis comes about as a result of changes to the proteins in muscle cells after death
    • Since no more oxygen reaches the muscle cells after death they will start to respire anaerobically, producing lactic acid
    • The accumulation of lactic acid decreases the pH in the muscle cells, denaturing the enzymes that produce ATP
    • Without ATP the myosin heads cannot be released from the actin filaments, locking the muscles in a contracted state
      • Muscles contract due to the action of two protein filaments; myosin and actin
      • The binding of myosin heads to actin proteins followed by the bending of the myosin heads causes muscle contraction
      • ATP is required to allow the myosin heads to detach from the binding sites on actin
    • This leads to the stiffness that is the main characteristic of rigor mortis
  • Rigor mortis will begin in the smaller muscles of the head and end in the larger muscles of the lower body, meaning that forensic experts can determine TOD by the progress of rigor mortis through the body
    • Rigor mortis would have taken place in every muscle between 12 and 18 hours after death, but will wear off again after about 24 to 36 hours from TOD
  • The process is affected by the level of muscle development and the temperature of the surroundings
    • Higher temperatures will speed up the rate of rigor mortis

Exam Tip

Remember that the time of death is only an estimate, it cannot be pinpointed exactly. There are so many factors that will affect a dead body that it is nearly impossible to provide a time of death that will be precisely accurate. Keep this in mind when answering questions on this topic.