Biocultural Taphonomies: Teasing apart taphonomic filters in bioarchaeology

Categories PGR, Research

I never learn. Which is ironic considering the sector I work in. Whenever I get asked if I’d give a paper at a conference, I always think “Oh, that’s ages away, literally months away, I’m busy now but it’ll be fine by then…”. But it never is. Such was the case when I was asked by Sabrina Agarwal and Trent Trombley if I’d give a paper on burning at a special symposium of the American Academy of Biological Anthropology in March this year. Unfortunately I wasn’t able to travel there in person, and so I wrote the paper and it was read, expertly so I hear, by Sabrina. I am still to determine whether she used the English accent that I requested. The paper was a bit too long, which was entirely my fault as I never write scripts for talks and so had no idea how many words filled 15 minutes, didn’t bother asking for advice and woefully over-estimated. So here is the unabridged version. The Director’s Cut, if you will. I’ve added some comments for context, and do feel free to use this in your teaching, seminars or as your summer beach reading…


Towards a bioarchaeology of cremation [This is the title. Great start, hey.]

This abstract is part of the symposium ‘Biocultural Taphonomies: Teasing apart taphonomic filters in bioarchaeology’. This presentation introduces the factors which influence the response of bone to burning. Extensive recent research in both the archaeological and forensic arenas has expanded the boundaries of the study of burned bodies, supporting a more nuanced understanding of human behaviour. This presentation reviews the synergistic impacts of taphonomy and thermal alteration on the preservation of skeletal remains, and the ways in which these introduce complexities to anthropological interpretation.

The skeleton undergoes very specific changes during heating and burning, although recent research has demonstrated the intimate interaction between the soft and hard tissues. Changes occur within the physical, chemical and crystalline structure of the hydroxyapatite, which historically have confounded interpretation. Current research on burned or cremated remains highlights the significance of both intrinsic and extrinsic factors for understanding how heat-induced changes manifest.

Using examples from my research projects in Europe and South America that cover a range of time periods, I will demonstrate how the methods that we have developed have allowed us to study increasingly complex cremation contexts. I will show how we are now able to combine heat-induced changes in the skeleton to reinterpret our perceptions of how the dead are treated. The bioarchaeology of cremation is moving into an exciting period of discovery and one in which the potential of this previously neglected archaeological resource can become more fully realised. [I wrote this Abstract months before I wrote the paper, and thus forgot what I’d put so it was a nice surprise when I came to write the actual paper.]

Burning is the most interesting taphonomic change to occur to the skeleton. [There. I said it. Suck it other taphonomists] Heat-induced change in bone is physiologically complicated, technologically challenging to study, and intellectually complex to interpret. The material is often highly fragmented and hard to recover and there are very few experts to work with across the globe. So, it is not hard to see why our understanding of funerary cremation contexts lags behind the more prosaic interred burials. For many years, it was assumed that burning was so destructive that there was no point in studying cremated bone, and indeed much material has been lost over this time as a result of this view. In fact, my own recent experiences have underlined how much this view is still held within archaeology, and more so in related fields like classics, history and the biological sciences. [My most spectacular example is a recent grant bid which got rejected because the reviewers did not believe you can tell anything from burned bone, despite me including reference to a load of papers on the subject.] But it is not true. Everything that we can learn about humans from unburned bone, we can learn from burned bone. It’s more challenging, yes, and we often need bespoke methods, but we can do it. We need to remember that our commonly used methods are predicated on unmodified bone dimensions. Therefore, a helpful way of conceptualising this problem is by thinking of our approach to population specificity. When we apply this idea to our anthropological methods, we usually think of sex or geographical differences. In the work by David Gonçalves where he and colleagues explored the development of methods of sex estimation from burned skeletons, a population specific approach was applied and was proven to be successful. [This is some of the most important work in underlining this point, with correct classification rates of around 80%] If we think of burned bone as a different population which requires its own relevant dataset, we can create osteoprofiles with significantly reduced inaccuracy.

On top of this, we know that burned bone can be recovered from a wide range of contexts that cover the archaeological, forensic and religious spheres. As such, the limited research that has been published can be found in the archaeological, forensic, ethnographic, medical, engineering and legal literatures – and rarely do they refer to each other. For many years, cremation contexts have been viewed as an alternative to inhumations, but this is unhelpful, and should be viewed as an activity in its own right. We should also note at this point that cremation is a process and not a singular action. It refers to activities that surround the burning of a body (both before and after) and not simply the application of fire.

With that in mind, it is worth considering how research in the field of burned bone studies has evolved over the years. [I started with a little history because it adds useful context to help us understand the nature of research going on now. Never have we had as much work being undertaken on burned bone as we have now.] The comparatively short period of development, compared to other aspects of bioarchaeology, means that it is easier to appreciate these developments. Initial publications from the 1950-1970s were highly descriptive. The likes of Baby in 1954 [classic paper] and Binford in 1963 [super-classic paper] provided detailed descriptions of sites which have, as a result, become classics in the field [told you]. The authors, without the benefit of years of research in heat-induced change to lean on, nevertheless provided some of the first attempts at interpreting funerary customs from archaeological and skeletal evidence from cremation contexts. It was soon realised that such interpretations would always remains limited unless we understood more about the changes that the skeleton exhibits as a result of burning. Thus, we see the emergence of studies in the 1980s which intentionally burn animal skeletons. These experimental studies emphasised the importance of actualistic results, that is, attempting to learn by mimicking the actual funerary processes as much as possible. The result of this approach was a detailed description of changes such changes, including colour change and fracture patterning. [The key theme I’m trying to emphasise with this potted history is not the study of cremations, but the study of burned bone and how that has been used to interpret the archaeological context. I rarely talk about pyre goods or other artefacts because I really have no idea what’s going on there.]

These were important studies, but what they didn’t allow was for archaeologists to answer more specific questions. Although one would describe these studies as scientific and experimental, they weren’t controlled in any real way – a consequence of trying to mimic real-world burning conditions. The result was that it was impossible to say, for example, whether temperature of burning or duration of burning was more important in causing change, or what the impact of clothing was, and so on. To address this, research from the mid-90s began to take burned bone experiments into the laboratory. Publications were produced that demonstrated to influence of individual variables from the cremation context, such as temperature, duration and anatomical region. Now the discipline was starting to understand the complex interaction between heat and bone – albeit against an on-going criticism that these experiments were not portable to the real world because they were so unrealistic. This is a criticism that it still levied at such research, and it is valid, although ignores that fact that few claim that they are supposed to be realistic. What they do, is allow for incremental growth in knowledge as study after study and result after result is layered on top of each other to provide a fuller understanding of heat-induced change. [This is where my own personal bias is most clear in my burned bone research. How can we interpret context from burned bone in the field if we don’t know what each component of bone or the body does when burned and how that might look to the investigator? We can’t. It’s madness. Madness I tell you. And I work in senior management, so I know madness when I see it.]

As we move into the noughties we see an increase in burned bone studies as a result of the momentum in the field. More publications means more academics reading about burned bone and more interest in its study. We can see that these experimental studies start to adopt increasingly advanced methods of analysis which provide ever more detailed observations and understanding of heat-induced change. This development is likely the result of increased interdisciplinary collaborations within universities whilst also an increase in accessibility (and reduction in cost) of these methods. It was an exciting time of rapid growth and genuine pushing of the boundaries of what was possible. Throughout this time we see that the discipline has matured sufficiently to allow for review studies to be published, which reflect upon our collective knowledge to date. This continues with the imminent publication of Ellingham et al’s book Burnt Human Remains: Recovery, Analysis, and Interpretation in October. [Sarah was one of my excellent PhD students and produced a number of key publications. She was really good at applying a rigorous statistical approach to her work and developed a range of predictive equations. She was also great and trialing new methods of analysis, which have been key to opening up new research questions. Each method tells us something slightly different about the bone, so when we combine them together, we get a really holistic dataset which allows for more nuanced interpretations. I’m super-proud of her and the work she’s doing now]

All of which brings us up to date. Where next for the development of the subject? For me, I think we can see a maturing of the questions we are asking. We know, on the whole, what happens to the skeleton as it burns. We know what to expect this taphonomic process to produce. What we seek to do now is take that understanding a step further; moving on from describing what we see in the archaeological context to stating how the assemblage was caused and then linking that to actual funerary practice. This understanding of the social context from the study of the biological context can be referred to as the bioarchaeology of cremation.

But before we explore some examples of this, it is worth taking a step back to discuss the basics of heat-induced change in bone. [I included this section because I was conscious that the delegates in the symposium probably weren’t as deeply immersed in the study of burned bone as I am (God knows what they’re doing with their lives then), and thus wouldn’t necessarily appreciate the key features we need to interpret. Plus this is a session on taphonomy and I hadn’t really mentioned taphonomic change yet.] As we know, bone is a complex material and all taphonomic factors that work on it produce complex changes. Burning is perhaps the most complex however, since it affects bone at all levels – physical, chemical and crystal. Broadly we see that bone works through four stages of transformation when burned – transformation, not destruction as it used to be termed since the latter implies that the resultant material is of no use anymore. The first stage, Dehydration, marks the loss of the water. The second, Decomposition, the loss of the organic phase. Then we have Inversion where the nature of the hydroxyapatite shifts and finally Fusion where the crystal structure reorganises and coalesces. These are not discrete phases, but rather a continuum, and all can occur on a single bone at once.

Many of the heat-induced changes that we talk about have been noted since those first publications on the subject by Binford, Gejvall, Van Vark and others over 50 years ago. Bone changes colour from its natural cream through to dark greys, black, light grey and then white. This reflects, not temperature as if often suggested, but rather the loss of the organic phase and is influenced by temperature, but also duration, oxygen levels and so on. The loss of the organic phase also results in measurable weight loss. This has been used to interpret MNI in cremation contexts, but is fraught with potential error resulting from, amongst other things, sex and age differences, incomplete collection, large error ranges, and so on. Fracturing occurs and has been used to differentiate between burning at the time of death and burning following a period of decomposition based on the shape of the fractures. Warping can be seen and has been the focus of ongoing debate. Initially felt to result from the build-up of hot gasses in the bone (which makes little sense since bone in highly porous) recent suggestions have included the contraction of collagen within the bone, or my current preference, the ‘softening’ of the bone as it progresses through the Inversion and Fusion stages of transformation. The surface of bone also changes as the inorganic phase become involved, leading to a smoothened surface with a reduction in porosity. Indeed, it becomes more ceramic in nature.

Noting this complexity, Thompson (2015) [I’ve heard he’s very good] recommended the use of an approach that classified these changes as either primary-level or secondary-level heat-induced changes. Changes such as fracturing and colour change were secondary-level, because they are manifestations of more fundamental change. This fundamental change focuses on the loss of the organic phase and the restructuring of the inorganic phase. These two changes cause all of the other changes we see, and so are referred to as primary-level changes. [Basically if we understand what is happening at the primary level, everything at the secondary level will make sense. This is why I focus my research there.]

What can be seen throughout all of this, and thanks to the history of research mentioned above, is that we are able to describe the causes of the heat-induced changes we see in burned bone, and therefore we can use the changes we see to make interpretations about the context of burning – from the remains themselves. This is the most important factor in determining whether we can use taphonomies meaningfully in bioarchaeology. Let’s explore this further with some examples.

Garrido-Varas and Intriago-Leiva (2015) described a forensic case in Chile [you can read the details, and view the images, in the book The Archaeology of Cremation] where a burned body was recovered but the investigators needed to know whether the burning had been associated with the context of death or had happened subsequently. The shape of the fracture patterns, the distribution of colour changes and the evidence of animal scavenging marks were noted. An understanding of their heat-induced aetiology allowed the anthropologists to conclude that the death and fire were unconnected. This is a common question. In unpublished work that our team undertook, we were tasked with a similar challenge but concerning ten-thousand-year-old bones from an extinct giant sloth recovered from a cave in the West Indies. Secondary-level changes, as used above, were not applicable since diagenetic changes of that timescale can mimic heat-induced changes. As such, an examination of crystal structure was required which determined that the bones had burned after decomposition. [These bastard sloth bones took four years to fully analyse and interpret. Separating diagenetic change from heat-induced change was so difficult to do that it led to the development of our new crystallinity measures because nothing else worked. It was also through this work that we realised the importance of using contemporaneous comparative material to successfully interpret burned bone – since it shows the natural level of diagenetic change to be expected at the site and deviation from this may be due to burning.]

This last point, that heat-induced changes and diagenetic changes are similar, is a key confounding factor in the investigation and interpretation of cremations in the past – especially if it unclear whether burning has occurred or not. The only solution to this is to collect contemporaneous, unburned bone material to act as a comparison. Thankfully the osteological changes due to burning make the bone essentially inert, and thus no significant diagenesis will occur after burning.

Hadrian’s Wall runs along the north of England and was an important Roman boundary, marking the gateway into the Empire. Many military sites were positioned along it, but the soldiers came from all across the Empire, with evidence of some having come from Syria. Wonderfully preserved wooden tablets from Vindolanda record such soldiers complaining about the grim, damp weather there [honestly, they are so impressive. Teesside colleagues have done some great work examining the importance of bacterial ecosystems on this preservation]. Vindolanda is less than an hour from where I work, and so I have some sympathy [we’re learning and having fun]. In this project, colleagues were interested to know how identity was expressed in the funerary remains of the dead soldiers. Our analysis (published in 2016) of the complex microstructural changes in the bone (which relied on comparisons to unburned material [see, I told you this was important – and you can read about it in the Journal of Archaeological Science: Reports]) allowed us to conclude that the deceased were all cremated using the same process, but that their remains were then placed in different containers. The consistency of the crystal change across all sites along the wall spoke to a shared cremation practice. Thus, soldier identity was two-fold – their military identity expressed through the manner of cremation, and their personal identity through their associated goods. This marked one of the first times that we were able to talk about the nature of social identity (or a duality of identity in this case) from a study of the burned biological bones themselves. [This is why this paper is important, because we are starting to overlay evidence types onto the interpretation of taphonomic change to explain, in essence, the decision-making of the long deceased.]

As methods become more refined, we are able to explore new questions and contexts. In 2021, Paba and colleagues published the results [awesome results, this is super-cool so keep reading] of their analysis of cremated fragments from a Middle Neolithic burial at Su Forru de is Sinzurreddus cave in Sardinia, Italy. This work formed part of the Trial by Fire special issue of the Journal of Archaeological Science: Reports which sought to expand how we interpret the use of fire in the past. [This was the result of a great conference run through UCL in 2019] In Paba’s work, the bodies were reconstructed through an anthropological analysis of the fragments. Then the regions of the body were subjected to examination of the crystal structure. With heat-induced changes recorded at high-resolution, it was possible to interpret the position of the body relative to the pyre during the act of burning and the fact that burning had occurred in a different location to deposition. The deceased had been placed on the left side in a crouched position – similar to the position of the deceased in the proceeding period when burial was common. Thus, we are able to see a connection between funerary practices; perhaps getting a glimpse into how funerary customs evolved gradually, rather than abruptly. [Oooooooohh! Look at what they did! You can read the details here. Did I neglect to say that I’m one of the co-authors? Yes. Is it still a brilliant piece of work? Fuck yes. Did this work get rejected from three journals because the reviewers didn’t think it was possible? Yes. Were they wrong. Embarrassingly so. Did I vow vengeance and retribution upon them and their houses? No. Really? Yes. Yes I did, or yes I didn’t? Yes.]

Let us move to a final example to end the talk with. Priscilla Ulguim is my indigenous Brazilian PhD student who has spent years studying her country, and for her doctoral research looked at the funerary practice in the southern Brazilian highlands [Priscilla is an incredible archaeologist who I love working with and who I’ve learned a lot from]. Evidence from mound and enclosure complexes across the southern Brazilian highlands indicates the development of a funerary ritual focused on cremation from c. AD 1000 until the contact period. These monumental earthwork enclosures form part of a highly structured landscape, consisting of circular or elliptical banks with plazas and central mounds. The mounds often contain burned bone, pyre-debris, and associated grave-goods, with evidence of feasting in the plazas. Evidence suggests occupation by the southern Jȇ, who are still represented in the region. She adopted bioarchaeological and bioanthropological methodologies, including a series of methods to investigate heat-induced change in bone, including analysis of bone crystalline structure using FTIR, variability in patterns of heat-induced colour change using digital colour quantification, and microscopic histological examination of the bone structure. [If you want to see your University finance staff go very pale, just tell them you want to ship all your expensive equipment over to Brazil for several months] These results were uniquely integrated with evidence from archaeology, ethnography and ethnohistory to interpret the significance of the individuals at these sites, defining important aspects of funerary practice in southern Jê archaeology. She concluded that articulated bodies were cremated outdoors after death in funeral pyres with high temperatures and for considerable periods of time. Cremation does not seem to have been restricted to a particular group, and was practiced by a variety of male and female biological individuals, observed in both primary and secondary cremated deposits. [A bioarchaeology of cremation, people.]

I hope that this talk has provided an insight into the taphonomic factors that burning causes [it has]. That the change to bone is significant and transformative, yet years of study mean that we understand such changes to the extent that we can interpret their origins from the human remains we study. Intellectual discussions and developments have gained momentum over the last ten years, and we head into an exhilarating period of bioarchaeology where we can genuinely seek to understand the social world from taphonomically altered bone. Burning is the most interesting taphonomic change to occur on the skeleton, and a bioarchaeology of cremation is upon us. [Unfortunately because I wasn’t able to give the talk myself, I wasn’t able to see the standing ovation, the tears, the fainting of those with a weak disposition, the throwing of flowers onto the stage, etc etc.]

I'm a Professor of Applied Biological Anthropology at Teesside University.