Although bone is seemingly never changing and durable, it is a tissue subject to constant change and prone to degradation throughout the life of an organism and beyond its death. These changes heavily affect the amount of information that remains available and visible to us and the way we should handle it. Especially in biomolecular archaeology, this is of crucial importance, since the degree of degradation usually dictates the suitability of the sampled material for analysis. Diagenesis represents the entirety of chemical, biological and physical processes occurring within a material or an organism during its conversion from the biosphere (sum of all ecosystems on earth) into the pedoshpere (outermost layer of earth), or in the case of archaeological samples its conversion from the biosphere to the state of preservation achieved by the time of excavation or analysis. Diagenesis as a process is generally understood in its separated components but as a whole phenomenon affecting the decomposing human body it remains a concept difficult to grasp in its entirety and variability.
There are a number of key criteria to consider when assessing the potential degree of degradation of human remains in an archaeological context: humidity, temperature, soil acidity, soil composition, biology of the individual at the time of death, postmortem body treatment and burial. Hydrology, temperature and soil composition will commonly be the deciding factor as to whether an environment is suitable for a good preservation and whether it will be aerobic (environment rich in oxygen) or anaerobic (environment poor or devoid of oxygen). This is important since most microorganisms involved in microbial degradation will be aerobic and therefore require an environment rich in oxygen to thrive, accordingly an anaerobic environment will generally have significantly less microbial activity, although a number of specialized anaerobic microorganisms will still be active. Each of the aforementioned criteria are often highly dependent on each other and have the potential for an immense impact on the final overall preservation achieved in the archaeological record within the conditions set by the combination of criteria present locally. This is true for the present and the past, since it cannot be assumed that the environmental conditions affecting an archaeological context have remained unchanged since its deposition (Guellil 2013; Hedges 2002; Nielsen-Marsh et al 2000). The exterior appearance of a bone or tooth has generally little to do with its overall preservation and can be very misleading. Instead of examining the exterior surface of a bone we have to take a good look at its microstructure. Typically, this is done by analysing the material histologically either via SEM or light microscope.
Bone is mainly composed of hydroxyapatite and type-1-collagen, with the mineral portion making up 60-70% of its weight and the remaining 20% organic portion being composed of 90% type-1-collagen. The preservation of hydroxyapatite and collagen are closely tied together in the bone matrix and form a structural symbiosis. Collagen breakdown is subject to some debate but is generally considered to be caused by bacterial collagenase and hydroxyapatite breakdown is caused by inorganic weathering, e.g. soil acidity. However, the structure of bone is such that if one is broken the stability of the other will be in jeopardy (Currey 2006; Dent et al 2004; Gill-King 1997; Guellil 2013; Trueman and Martill 2002; Weiner 2010). The loosening of the bone matrix caused by collagen leaching usually leads to a facilitated access to the bone for microorganism, who in turn will increase the porosity of the bone by tunneling through the already damaged and weakened matrix. Generally speaking these agents are divided into microbial and fungal degradation and leave very distinctive marks on the bone microstructure called microscopical focal destruction or MFD. After skeletonization the survival of bones is mostly dependant on inorganic weathering and mechanical stress. Ideally speaking a good preservation of skeletal remains is guaranteed in a neutral or slightly alkaline anaerobic cold environment (Dent et al. 2004; Jans et al. 2004, 87; Weiner 2010, 113-114).
The degree of degradation affecting bone is generally considered to be set at a very early stage of its deposition and seems to be subject to very little change after ca. 500 years and literature has been debating about the origin and characterisation of the agents affecting bone degradation. To predict anything when so many factors are prone to variability is a challenge that can only be addressed by collecting more data and gaining an understanding for the small links that hold the bigger picture together (Guellil 2013; Hedges 2002; Hollund et al. 2012; Trueman and Martill 2002). With new technology and screening techniques emerging, which are able to deal with sample degradation up to a certain level, we can now extract viable data from degraded materials, from which we would not have been able to sample in the past. However, the final yield itself and its quality will always depend on the degree of degradation and the depositional environment the sample source was subjected to.
About the Author:
Meriam Guellil is a Research Associate in the Biology Department at the University of York, working within the BioArCh Research Cluster. Her research focuses on Ancient DNA, BioInformatics and Taphonomy, working on a range of samples from Canada, Italy and the UK. Meriam completed a BA in Prehistoric Archaeology and Early History at the University of Vienna, Austria, followed by an MSc in Human Osteology and Funerary Archaeology at the University of Sheffield.
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