Building a Better Egg Timer?

化石和DNA证据表明，我们的物种智人（Homo sapiens）在近20万年前出现在非洲，但直到最近，我们认为的“现代行为”的许多迹象似乎直到5万年前才显现出来。这一关键时期的考古证据极其稀少，年代也不确定。在此期间发生的剧烈气候和环境变化可能是关键的影响，但由于缺乏可靠的定年方法，很难将不完整的人类记录与现有的连续全球气候记录或存在这些记录的区域记录联系起来。随着新的测年方法的发展和现代行为早期发生的证据的积累，例如海洋资源的开发和一些地点装饰或装饰物品的出现，情况开始发生变化。后者包括鸵鸟蛋被用作容器和装饰珠子的迹象。然而，目前这些人工制品都不能直接准确地确定年代。一种可以直接测定蛋壳和蛋壳年代的方法是氨基酸消旋化（AAR）地质年代学，它可以测量蛋白质的衰变。由于衰变速率受时间和温度的影响，因此可以通过与其他样品的比较推断出相对年龄。确定绝对日期更具挑战性。为了提供一个可靠而精确的年龄估计，必须了解驱动衰变反应的机制。我们一直在开发一种氨基酸地质年代学的改进方法，称为“晶体内蛋白质分解”（IcPD），只从外壳的特定部分中分离和分析蛋白质，这些蛋白质被困在外壳晶体中。这有效地形成了一个蛋白质的“时间胶囊”，在其中，蛋白质衰变的产物和反应物的浓度变化应该使我们能够区分反应途径。过去，我们受限于对衰变的粗略测量，但技术的进步，特别是质谱法，现在使我们能够相当详细地观察生成物和反应物。我们计划探索鸵鸟蛋壳碎片的反应途径，这是最终的“蛋定时器”。初步结果揭示了包裹在古代贝壳中的主要蛋白质（称为struthiocalcin）的衰变途径的第一个证据。蛋白质首先分成三块，每一块都从一端“啃掉”。通过结合模型系统（合成肽、加热壳和壳蛋白）和从南非埃兰兹湾洞穴的分层和年代确定的考古序列中恢复的壳片段，我们可以探索这种初始模式是如何发展的。一旦对衰变有了更全面的了解，我们就会建立一个衰变路径的模型（它可以确定关键反应的温度敏感性）。然后，我们将应用这个模型来估计在南非Pinnacle Point丰富的考古序列中发现的蛋壳碎片的年龄，以完善这些早期现代人类遗址的时间框架。但是鸵鸟蛋壳不仅仅是鸡蛋计时器——如果我们观察蛋壳的同位素组成，这也揭示了气候和环境的信息。因此，我们可以建立一个可靠的、直接确定年代的植被变化记录，更重要的是，该地区的干旱情况。这些数据可以与更长的气候记录联系起来，以建立一个更完整的人类行为变化与环境变化之间关系的图景。

Fossil and DNA evidence suggests that our species, Homo sapiens, emerged nearly 200,000 years ago in Africa, but until recently it seemed that many of the signs of what we would consider 'modern behaviour' were not evident until after 50,000 years ago. The archaeological evidence for this crucial period is exceedingly sparse and poorly dated. The dramatic climate and environmental changes that occurred during this period were likely to be critical influences, but in the absence of reliable dating methods it is difficult to link the patchy human record with existing continuous global climate records, or regional records where they exist. The picture has begun to change with the development of new dating methods and accumulating evidence for the early occurrence of modern behaviours, such as marine resource exploitation and the appearance of decorated or decorative objects in several sites. The latter include indications that ostrich eggs were used as containers and as decorative beads. However, at present none of these artefacts can be dated directly and precisely. One method that can be used to date shells and eggshells directly is amino acid racemization (AAR) geochronology, which measures the decay of proteins. Because the decay rate is driven by time and temperature, the relative age can be deduced by comparison with other samples. Determination of an absolute date is more challenging. In order to provide a robust and precise age estimate, the mechanisms that drive the decay reaction must be understood. We have been developing a refinement of amino acid geochronology, called 'Intra-crystalline Protein Decomposition' (IcPD), isolating and analysing only the protein from a particular fraction of shells, where the proteins have been trapped within the shell crystals. This effectively forms a protein 'time capsule', within which changing concentrations of the products and reactants of protein decay should enable us to pick apart the reaction pathways. In the past we have been limited by crude measurements of decay, but technological advances, notably in mass spectrometry, now enable us to observe both products and reactants in considerable detail. We plan to explore the reaction pathways in ostrich eggshell fragments, the ultimate 'egg-timer'. Preliminary results reveal the first evidence of a decay pathway in the main protein entrapped in ancient shell, called struthiocalcin. The protein first appears to split into three pieces and each piece is 'nibbled away' from one end. By using a combination of model systems (synthetic peptides, heated shells and shell proteins) and shell fragments recovered from the stratified and well-dated archaeological sequence in Elands Bay Cave, South Africa, we can explore how this initial pattern develops. Once a more complete picture of decay is understood, we will put together a model of the decay path (which identifies the temperature sensitivity of key reactions). We will then apply this model to estimate the age of eggshell fragments found in a rich archaeological sequence at Pinnacle Point, South Africa, to refine the chronological framework of these early modern human sites. But the ostrich eggshells are more than just egg-timers - if we look at the isotopic composition of the eggshells, this also reveals information about climate and environment. Thus we can establish a robust, directly dated record of changes in vegetation and importantly, in aridity, for the region. This data can be linked to longer climate records to build up a more complete picture of the relationships between changing human behaviour and changing environments.