Bacterial bioerosion of bones is a post-skeletonisation phenomenon and appears contingent on soil burial

The post-mortem changes that take place during the decomposition of corpses are extremely complex and have been the subject of considerable research because of their obvious importance to forensic scientists. Considerable advances have been made in understanding the interplay of the various factors that influence decay, and the timescales over which they operate. Similarly, changes to the chemistry and microstructure of bones, post-skeletonisation, are of intense interest to archaeological scientists and the past few decades has seen tremen-dous advances in our understanding of how post-burial diagenesis may impact endogenous evidence within ancient and fossil bones. In recent years it has been postulated that the early stages of putrefaction of a body have a profound effect on the microstructure of bones and that these changes persist over archaeological timescales. One of the most important mechanisms for post-depositional degradation of bones is bacterial bioerosion. The question of the origin of the bacteria responsible, and why some archaeological bones are bioeroded while others are not, has received considerable attention in recent years. The assertion by some that the bacteria originate in the gut, and infiltrate bone tissues via the vascular network during the putrefactive stages of decay, has led to a number of claims as to what can be inferred by the presence, or absence, of bacterial tunnelling in bones. These claims include evidence for human mummification, identification of stillborns and infanticide, and animal sacrifices. Here we present evidence from SEM histological examinations of bones from field experiments using intact pig carcasses interred in brick lined tombs; a forensic case of a body exposed on a concrete floor for ten years; together with de-fleshed bones from freshly slaughtered cows buried in tropical soils for 1-10 years. None of the decomposing pig carcasses, nor the human corpse, showed any signs of bacterial tunnelling in their bones. By contrast, the de-fleshed bones buried directly in soils showed evidence of tunnelling after only one year and considerable tunnelling after ten years. Our results cannot support the "enteric hypothesis" that the bone tunnelling microorganisms that degrade buried bones originate in the gut microbiota but rather suggest that gut bacteria play no role in the post-mortem degradation of buried bones. It therefore follows that no conclusions about post-mortem treatment of human or animal corpses can be deduced from the presence, or absence, of microbial tunnelling in skeletonised remains from an archaeological (or forensic) context.