Does ancient human dental enamel record short-term seasonality? Investigating time-lines in teeth with computed tomography and oxygen isotopes.

A human tooth crown holds a mini-archive of information recording the diet, climate and residence of its owner. This information allows us to reconstruct what an individual was eating, and what the environmental conditions were in the past. In particular, if we can construct a temporal sequence for an individual we can also determine how they moved around and utilized this environment, and what seasonal changes in temperature were experienced. We know this can be done from the enamel of the molar teeth of cattle and sheep where there is a relatively straightforward 'time-line' spanning one or more years from the top of the tooth towards the root. These estimates for tooth crown development have been confirmed by recent isotopic studies that demonstrate seasonally distinct signatures rather than an averaged value. Human teeth, however, are very different in how they form and in their final, bulbous shape. Although the sequence of human tooth formation and eruption is well established, the pattern of enamel mineralization is very poorly understood. An initial organic scaffold of enamel is laid down over regular time periods producing a visible laminated structure (Lines of Retzius). The mineralization process, in which enamel forms within and eventually replaces the organic framework, is extremely complex and may progress spatially in several directions at once. Thus human enamel does not closely follow the 'time-line' of organic deposition, or at least, not in a straightforward chronological manner. It has been proposed that enamel mineralizes from the biting edge down, from the inside out, from the outside in, along the lines of organic matrix deposition and that thin enamel mineralizes before thick. We need to know the length of time that mineralization takes to complete, whether enamel simply records an averaged value for the whole period throughout its depth and across its surface, and if, where, and in what direction, the passage of time is recorded. Understanding this pattern is essential if we are to extract time-series information from human tooth crowns, and to interpret this data rigorously. The problem has been impossible to address in living children without repeated X-ray or large-scale extraction of developing teeth. Since these are ethically unacceptable, pigs have been used as the closest human proxies. A new approach has been identified. Archaeological skeletal collections contain many children who died whilst their teeth were still developing. The processes of tooth mineralization can be investigated using computed tomography (CT), even for teeth encased within the jaw. The method is non-destructive, repeatable and highly sensitive to differences in tissue density. A pilot study showed that fully mineralized enamel can be differentiated from incompletely mineralized enamel within a single tooth, and the difference quantified and viewed in 3D. This study will obtain multi-slice, CT scans from developing tooth crowns (incisors, canines, premolars and molars). In total, 408 teeth from over 100 juvenile skeletons curated at the University of Bradford will be scanned to investigate progressive stages of mineralization in each of the 10 selected tooth categories. Using specialized MIMICS software, 3D coloured images and animations will be produced to track the advancing front of mineralizing enamel. Where potential 'time-lines' are identified, we will test them in fully developed modern and ancient teeth using a laser to sample along the length of the line. If the passage of time is recorded in the enamel, the oxygen isotope ratios along the line should vary in a regular manner because they are sensitive to changes in climate, particularly temperature differences between summer and winter. However, if they do not vary or if variation is random, irregular and not reproducible between teeth developing at the same time in a single individual, short-term seasonality may not be preserved in human enamel.