Morphological evolution and the Cambrian Explosion - a 550 million year view

现存动物的多样性可以分为大约30组，称为门，这个术语指的是拥有独特的身体计划。所有这些身体计划首次出现在化石记录中的地质眨眼之间，在一个被命名为寒武纪爆炸（约5.5亿至5.2亿年前）的事件中。在这段时间里，不同的动物，如蠕虫，水母，甚至我们的远亲，都可以作为化石被发现，与更古老的岩石形成对比，那里的大多数生命证据都是微观的。这一时期的古代动物化石很难分类，一些科学家此前认为寒武纪代表了动物进化的“实验”阶段，动物进化的形式从未再出现过。寒武纪大爆发被认为是地球历史上一个完全独特的事件，进化变化发生得更快，规模比任何其他时间都大。不同体型的范围被称为差异，以前的一些研究提出，这种差异在寒武纪达到顶峰，然后随着时间的推移而灭绝。其他研究则将寒武纪化石视为生物体进化的先行者，填补了现今活着的群体之间存在的身体计划之间的空白。然而，对寒武纪这些进化模式的研究通常集中在一组狭窄的动物群体上，最常见的是节肢动物（蜘蛛，蜈蚣及其亲属），或者只考虑寒武纪生物和今天活着的生物的证据，缺少来自5亿年地球历史的证据。自从寒武纪大爆发结束以来，动物们已经从根本上改变了他们的身体计划，通常是在迁移到新的，有时是极端和恶劣的环境时，例如从海洋过渡到陆地上的生命或热液喷口的殖民。在这个项目中，我将使用一组被称为Spiralia的动物来研究从寒武纪爆发到现在，动物形式的多样性是如何形成的。螺旋藻种类繁多，包括蚯蚓和蜗牛等熟悉的群体，但也包含一系列主要生活在海洋中的更不寻常的形式。Spiralians占动物身体计划的一半以上，并且从寒武纪大爆发的最早阶段开始就有很好的化石记录。总之，这些事实突出了这组生物体作为理解动物差异进化的理想研究系统。本研究课题将解决以下研究问题：Q1。在5.4亿年的螺旋进化过程中，形态空间占据（差异）的模式是如何变化的？Q2.是什么样的进化模式沿着谱系产生了这些形态差异的模式？我将通过收集寒武纪和奥陶纪早期（约5.42 - 4.8亿年）保存完好的古老化石的新数据来完成这一目标，我将联合收割机与活物种的数据结合起来，构建称为进化图的进化图。这些就像一个家谱，描绘了不同类型生物之间的关系。建立这样一个螺旋虫的生命树将是关键的第一步，需要使用来自DNA序列、化石和活体动物解剖的信息。由此产生的进化树不仅仅是图表，因为它们允许调查进化如何随着时间的推移而进行，使我能够估计过去进化变化的速度（“速率”），以及发生的变化总量。这将使我了解新的形态变化是如何随着时间的推移从寒武纪到今天产生的。最后，它将让我了解寒武纪大爆发和随后的事件如何塑造了今天壮观的生物多样性。

The diversity of living animals can be placed into about 30 groups called phyla, a term that refers to the possession of a distinct body plan. All these body plans first appear in the fossil record in a geological blink of an eye, in an event that has been named the Cambrian explosion (about 550-520 million years ago). During this time, animals as different as worms, jellyfish and even our distant relatives can be found as fossils, contrasting with more ancient rocks where most evidence of life is microscopic. The fossils of ancient animals from this time have been difficult to classify, with some scientists previously arguing that the Cambrian represents an 'experimental' phase in animal evolution, with types of animals evolving forms that have never been seen again. The Cambrian Explosion has been considered an entirely unique event in the history of the planet, with evolutionary change occurring faster and with greater magnitude than at any other time. The range of different body forms is referred to as disparity, and some previous studies have proposed that this disparity peaked in the Cambrian and was then educed over time by extinction. Other studies have instead viewed Cambrian fossils as the evolutionary antecedents of living organisms, filling in the gaps that exist between the body plans of groups alive today. However, investigations into these patterns of evolution during the Cambrian have typically focused either on a narrow set of animal groups, most often arthropods (spiders, centipedes and their relatives), or have only considered evidence from Cambrian organisms and those alive today, missing evidence from the intervening 500 million years of Earth History. Since the Cambrian Explosion ended, animals are known to have radically modified their body plans, often when moving to new and sometimes extreme and hostile environments such as the transition from the ocean to life on land or the colonisation of hydrothermal vents. In this project I will use the group of animals called Spiralia to investigate how the diversity of animal forms was shaped since their origin in the Cambrian Explosion to the present. Spiralians are incredibly diverse, encompassing familiar groups like earthworms and snails but also containing a bewildering array of more unusual forms that primarily live in the ocean. Spiralians account for over half of animal body plans and have an excellent fossil record from the earliest phase of the Cambrian explosion onwards. Together these facts highlight this group of organisms as the ideal study system for understanding the evolution of animal disparity. This research project will address the following research questions:Q1. How have patterns of morphospace occupation (disparity) varied through 540 million years of spiralian evolution?Q2. What patterns of evolution along lineages gave rise to these patterns of morphological disparity?I will accomplish this by collecting new data from exceptionally preserved, ancient fossils from the Cambrian and early Ordovician Periods (about 542-480 million years old) which I will combine with data from living species to construct evolutionary diagrams called phylogenies. These are like a family tree and depict the relationships between different types of organisms. Building such a tree of life for spiralians will be a key first step, and will require using information from DNA sequences, fossils and the anatomy of living animals. The resulting evolutionary trees are more than just diagrams as they allow investigation of how evolution has proceeded through time, allowing me to estimate the speed ('rate') of evolutionary changes in the past, and the total amount of change that happened. This will allow me to understand how new morphological variation has been generated over time from the Cambrian to the present day. Finally, it will allow me to understand how the Cambrian Explosion and subsequent events shaped the spectacular biodiversity alive today.