Decoding the fossil record of embryology at the dawn of animal evolution

胚胎学是研究发育的学科，它是揭示动物进化史的关键，也是揭示发育模式的变化如何产生生命之树分支事件的关键。古生物学可以为这些假设提供唯一的直接检验，但传统上，由于缺乏保存在化石记录中的胚胎学数据，古生物学在这些争论中一直保持沉默。随着在主要动物群首次出现时沉积的岩石中发现丰富多样的化石胚胎组合，进化生物学和古生物学的前景立即改变。这些化石对于理解进化史上这一关键时期的发育进化具有巨大的潜力，但由于难以区分保存下来的生物结构和后来的地质矿化，而且很难理解单个化石胚胎的3D解剖结构，更不用说从一个胚胎阶段到另一个胚胎阶段的发育过程中发生的变化了。这些挑战导致了对化石胚胎属于哪个动物群的不同解释--在得出进化结论之前，必须结束这一争论。事实上，情况非常糟糕，尽管化石胚胎通常保存完好，包括细胞和亚细胞细节，但一些科学家甚至怀疑它们属于动物，而宁愿将它们解释为巨型细菌。我们对化石的解释将通过使用高分辨率版本的医学CT扫描仪进行分析来辅助，该扫描仪由粒子加速器提供动力，并允许我们基于虚拟计算机模型分析化石胚胎的3D解剖结构，包括其内部结构。这使我们能够分析小于百万分之一毫米的胚胎解剖细节，揭示细胞运动甚至亚细胞结构的细节，这些细节对于确定胚胎学，亲和力以及这些5亿年前化石的进化重要性是必要的。我们的CT扫描还揭示了保存的生物和地质结构的化学成分差异，有助于解释化石。我们将确定这些化学差异的基础，并使用结果来判断来自同一地质层的有争议的化石胚胎和成体之间的相互冲突的解释。进行这些实验是为了更好地了解已知的化石胚胎，同时也是为了揭示化石胚胎和幼虫的解剖结构，这些化石胚胎和幼虫对科学来说是全新的，在我们的国际团队的收藏中，几乎包括化石胚胎领域的每一位领先科学家。我们还将进行腐烂实验，以了解（i）巨型细菌和（ii）动物胚胎中的亚细胞结构的腐烂。这些数据将用于指导我们对化石中生物结构的解释，使我们能够决定细菌或动物胚胎模型的解释最适合。最终，我们的目标是揭示化石胚胎的胚胎学，以此作为找出它们最接近的活体亲属的一种方式，以及胚胎学的模式是否在化石胚胎和它们的活体亲属之间进化。总的来说，这些答案将揭示胚胎进化在动物进化史上最基本、最具争议的事件之一中的作用：我们今天看到的现代动物群体的建立。

Embryology, the study of development, is pivotal to unravelling the evolutionary history of animals, and how changes to patterns of development have produced the branching events in the Tree of Life. Palaeontology can provide the only direct tests of such hypotheses but, traditionally, it has been silent in such debates because of a dearth of embryological data preserved in the fossil record. The outlook for evolutionary biology and for palaeontology was changed at once with the discovery of rich and diverse assemblages of fossil embryos in rocks deposited at the time that major animal groups were first emerging. These fossils have a huge potential for understanding developmental evolution at this critical episode of evolutionary history, but it is largely unrealised because of difficulties in distinguishing preserved biological structure from later geological mineralization, and because it is difficult to understand the 3D anatomy of an individual fossil embryo let alone the changes that took place during development from one embryological stage to another. These challenges have led to wildly differing interpretations of which animal group the fossil embryos belong to - a debate that must be concluded before evolutionary conclusions can be made. In fact the situation is so bad that although the fossil embryos are often beautifully preserved, including cellular and sub-cellular details, some scientists even doubt they belong to animals, preferring instead to interpret them as giant bacteria. Our interpretation of the fossils will be aided by analysis using a high resolution version of a medical CT scanner, which is powered by a particle accelerator and allows us to analyse the 3D anatomy of a fossil embryo, including its internal structure, based on a virtual computer model. This allows us to analyse details of an embryo's anatomy that are less than a millionth of a millimetre, revealing details of cell movements and even sub-cellular structure that are necessary to determine the embryology, affinity and, ultimately, the evolutionary importance of these half billion year old fossils. Our CT scans also reveal differences in the chemical composition of preserved biological and geological structures, aiding the interpretation of the fossils. We will determine the basis of these chemical differences and use the results to judge between conflicting interpretations of controversial fossil embryos and adults from the same geological layers. These experiments will be conducted to better understand known fossil embryos, but also to uncover the anatomy of fossil embryos and larvae that are completely new to science, in the collections of our international team which includes almost every leading scientist in the field of fossil embryos. We will also undertake rotting experiments to understand the decay of (i) giant bacteria and, (ii) the subcellular structures in animal embryos. These data will be used to guide our interpretation of the biological structure in the fossils, allowing us to decide whether the bacteria or animal-embryo model of interpretation fits best. Ultimately, our aim is to uncover the embryology of fossil embryos as a way of working out their closest living relatives, and whether the pattern of embryology has evolved between the fossil embryos and their living relatives. Collectively, these answers will reveal the role of embryological evolution in one of the most fundamental and most-debated episodes in animal evolutionary history: the establishment of the modern animal groups that we see about us today.