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

胚胎学，一门研究发育的学科，对于揭示动物的进化史，以及发育模式的变化如何在生命之树上产生分支事件至关重要。古生物学可以为这些假设提供唯一的直接检验，但是，由于化石记录中保存的胚胎学数据的缺乏，古生物学在这类争论中一直保持沉默。进化生物学和古生物学的前景立刻发生了变化，因为在主要动物群体首次出现时沉积的岩石中发现了丰富多样的化石胚胎组合。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，但由于很难将保存下来的生物结构与后来的地质成矿区分开来，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发育阶段到另一个胚胎发育阶段发生的变化，因此在很大程度上尚未实现。这些挑战导致了对化石胚胎属于哪个动物群体的截然不同的解释——在得出进化结论之前，必须先结束这场争论。事实上，情况是如此糟糕，以至于尽管化石胚胎通常保存完好，包括细胞和亚细胞的细节，一些科学家甚至怀疑它们属于动物，而宁愿把它们解释为巨大的细菌。我们对化石的解释将通过使用高分辨率的医学CT扫描仪来辅助分析，该扫描仪由粒子加速器提供动力，允许我们分析化石胚胎的3D解剖，包括其内部结构，基于虚拟计算机模型。这使我们能够分析不到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，这些细节对于确定这些5亿年前的化石的胚胎学、亲和力以及最终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的差异，有助于解释化石。我们将确定这些化学差异的基础，并利用结果来判断来自同一地质层的有争议的化石胚胎和成人的相互矛盾的解释。这些实验将进行，以更好地了解已知的化石胚胎，也揭示化石胚胎和幼虫的解剖结构，这对科学来说是全新的，在我们的国际团队的集合，其中包括几乎所有领先的科学家在化石胚胎领域。我们还将进行腐烂实验，以了解(1)巨型细菌和(2)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是否最合适。最终，我们的目标是揭示化石胚胎的胚胎学，以此来研究它们最近的现存亲属，以及胚胎学的模式是否在化石胚胎和它们的现存亲属之间进化。总的来说，这些答案将揭示胚胎学进化在动物进化史上最基本和最具争议的一段时期中的作用：我们今天所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.