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

人类的牙冠保存着一个小型的信息档案，记录了它的主人的饮食，气候和居住地。这些信息使我们能够重建一个人吃什么，以及过去的环境条件。特别是，如果我们可以为一个个体构建一个时间序列，我们也可以确定他们如何移动和利用这个环境，以及经历了什么样的季节性温度变化。我们知道这可以从牛和羊的臼齿的釉质中完成，那里有一个相对简单的“时间线”，从牙齿的顶部到根部跨越一年或多年。最近的同位素研究证实了牙冠发育的这些估计，这些研究表明季节性不同的签名，而不是平均值。然而，人类牙齿的形成方式和最终的球根状形状非常不同。虽然人类牙齿形成和萌出的顺序已经很好地建立，但对釉质矿化的模式却知之甚少。釉质的初始有机支架在规则的时间段内铺设，产生可见的层压结构（Retzius线）。矿化过程，其中釉质形成并最终取代有机框架，是非常复杂的，可能会在空间上同时在几个方向上进行。因此，人类的牙釉质并不紧密地遵循有机沉积的“时间线”，或者至少不是以直接的时间顺序方式。有人提出，釉质矿化从咬边向下，从内到外，从外到内，沿着有机基质沉积的线条，薄的釉质在厚的之前矿化。我们需要知道矿化完成所需的时间，釉质是否只是记录了整个深度和表面的整个时期的平均值，以及时间的流逝是否、在哪里、在什么方向被记录下来。如果我们要从人类牙冠中提取时间序列信息，并严格解释这些数据，那么理解这种模式是必不可少的。如果不反复进行X光检查或大规模拔除发育中的牙齿，这个问题就不可能在活着的孩子身上得到解决。由于这些在伦理上是不可接受的，猪被用作最接近人类的代理人。已经确定了一种新的办法。考古骨骼收藏品中包含许多儿童，他们在牙齿仍在发育时死亡。牙齿矿化的过程可以使用计算机断层扫描（CT）进行研究，即使是在颌骨内的牙齿。该方法是非破坏性的，可重复的，并且对组织密度的差异高度敏感。一项初步研究表明，在一颗牙齿中，完全矿化的釉质可以与不完全矿化的釉质区分开来，并且可以在3D中量化和观察差异。本研究将获得发育中牙冠（门牙、犬齿、前磨牙和磨牙）的多层CT扫描。总共将扫描来自布拉德福德大学100多个青少年骨骼的408颗牙齿，以调查10个选定牙齿类别中每一个的矿化进展阶段。使用专门的MIMICS软件，将产生3D彩色图像和动画，以跟踪矿化釉质的前进前沿。在潜在的“时间线”被确定，我们将测试他们在充分发展的现代和古代牙齿使用激光采样沿着长度的线。如果时间的流逝被记录在牙釉质中，那么氧同位素比率沿着这条线应该会有规律地变化，因为它们对气候的变化很敏感，特别是夏季和冬季之间的温差。然而，如果它们不变化，或者如果变化是随机的，不规则的，并且在同一时间在单个个体中发育的牙齿之间不可再现，则短期的季节性可能不会在人类釉质中保留。

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.