Last week I discussed a way of preserving bodies almost indefinitely in some cases: embalming. On the other side of this is decay, the process of bodily decline and biological breakdown of the flesh. If you’ve ever watched any of the forensics crime shows, you know that understanding decay and changes in the body can be a key factor in determining when the individual died and how the body was treated after death. But its also important for archaeologists dealing with remains that are ancient.
First, let’s look at the early stages of decay. I won’t go into the gooey details, but following death the flesh of the body goes through five stages. The first stage consists of the ‘mortis‘ phases. The blood isn’t being pumped through the body so due to gravity it pools in certain areas, and this is known as livor mortis. Shortly after this, the muscular tissue becomes rigid and incapable of relaxing, a state called rigor mortis. Next the body loses heat and cools in a process called algor mortis. Second, the body goes through bloat, in which means that microbes are rapidly growing and forming gases within the body. This is also usually when bugs and insects begin to feed and reproduce on the remains. In the third stage there is rapid loss of mass due to insect feeding and natural purging of fluids due to decomposition. Advanced decay is the fourth stage, and there is little left of the body at this point. Finally, the last stage is skeletonization when no flesh remains. But this isn’t the end- bones are also subject to continued decay, the study of which is known as taphonomy and is extremely important for archaeologists.
Taphonomy is the study of what happens to remains after the death of the living creature. Usually, the focus of these studies for archaeologists is to determine why there is a bias in the archaeological record (i.e. why certain bones preserve in certain conditions and others do not). Human remains do not always appear as clean bones in the ground. Sometimes they are badly degraded, not all bones may have survived through time, and in certain conditions the skeleton is no longer present. Taphonomic studies try to determine why certain bones are present, and others are not. Here are two examples. The left hand side is an Anglo-Saxon burial where the bones are fairly well preserved. Obviously we are missing some ribs and the ends of the long bones did not preserve well, however it is intact enough that we can gain insight into who this person was based on the remains. The right hand side shows the complete opposite where no bones at all were preserved in the burial. This means we cannot learn anything about the person biologically, and must rely on the grave goods alone. Both are from the same country and same time period- so why such a difference in preservation? This is what taphonomic studies hope to answer.
There can be extrinsic (external) factors, or intrinsic (internal) factors that effect preservation. Soil characteristics have a major effect on whether bone is preserved. As soil temperature and acidity increases in the soil, it is less likely that the bone will be preserved. Types of soil also affect whether bone is preserved: sand can preserve bone very well, whereas salt and chalk are destructive to bone (for more on these external factors see this article by Baxter). Other factors include water, which in an anaerobic environment causes preservation and aerobic causes destruction. Internal factors like the porosity of the bone can also affect preservation. The bones of the very old or very young are often more porous due to osteoporosis or lack of development, respectively. In cemeteries we rarely find infant remains, which could be due to the fact that they don’t preserve well or that they are buried elsewhere (see this post on an infant burial site for more information on this occurrence).
Cultural practices can also affect how well bone is preserved. For example, if the body is buried in a bog it may be perfectly preserved, whereas if it is buried in acidic soils it could be completely destroyed. Burial disturbances such as exhumation, additional interments, and other intrusive actions can lead to bone being more susceptible to decay. A study done by Lieverse et al. (2006) found that “primary burials exhibited higher completeness and increased articulation compared to secondary burials”, and those that were primary burials were better preserved. Factors such as burial location, depth of burial and container also were important in determining how well the bones preserved. They concluded that cultural practices were a prime agent of bone destruction or preservation. Those bones which were disturbed by human agency were less well preserved than those which weren’t disturbed by cultural action.
Taphonomy drastically effects what we find and how much we can analyze. It is important to know why bones aren’t as well preserved as others, and what factors may have erased them completely. While this isn’t an exhaustive list of what can affect bones, it is a good beginning!
Lieverse, A., Weber, A., & Goriunova, O. (2006). Human taphonomy at Khuzhir-Nuge XIV, Siberia: a new method for documenting skeletal condition Journal of Archaeological Science, 33 (8), 1141-1151 DOI: 10.1016/j.jas.2005.12.001