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

活着的动物的多样性可以分为大约30个类群，称为门，这个术语指的是拥有独特的身体规划。所有这些天体计划在地质上一眨眼的功夫就首次出现在化石记录中，这一事件被称为寒武纪大爆炸(约5.5亿-5.2亿年前)。在这段时间里，像蠕虫、水母甚至我们的远亲这样不同的动物都可以作为化石被发现，这与更古老的岩石形成了鲜明对比，在那里大多数生命的证据都是微小的。这一时期的古代动物化石很难归类，一些科学家此前认为，寒武纪代表着动物进化的“实验”阶段，动物的类型进化出了从未有过的形式。寒武纪的爆发被认为是地球历史上一次完全独特的事件，其进化变化发生得比以往任何时候都要快，规模也更大。不同身体形态的范围被称为差异，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被灭绝所消除。相反，其他研究将寒武纪化石视为生物进化的先驱，填补了当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要么集中在少数动物群体上，最常见的是节肢动物(蜘蛛、蜈蚣及其近亲)，要么只考虑寒武纪生物和今天活着的人的证据，而忽略了其间5亿年地球历史的证据。自寒武纪大爆发结束以来，众所周知，动物已经从根本上改变了它们的身体规划，通常是在迁移到新的、有时是极端和恶劣的环境时，比如从海洋过渡到陆地上的生命，或者热液喷口的殖民。在这个项目中，我将使用名为Spiralia的动物群体来研究动物形式的多样性是如何从寒武纪大爆发到现在的形成的。螺旋虫是极其多样化的，既包括人们熟悉的种类，如蚯蚓和蜗牛，也包括一系列令人眼花缭乱的更不寻常的形式，主要生活在海洋中。从寒武纪爆发的最早阶段开始，螺旋藻就占据了动物身体计划的一半以上，并拥有出色的化石记录。总而言之，这些事实突出表明，这组有机体是理解动物差异进化的理想研究系统。本研究项目将解决以下研究问题：Q1。在5.4亿年的螺旋式进化过程中，形态空间占据(差异)的模式发生了怎样的变化？是什么进化模式导致了这些形态差异的模式？我将通过收集寒武纪和早奥陶世(约5.42亿-4.8亿年前)保存完好的古化石的新数据来完成这一点，我将与现有物种的数据结合起来，构建称为系统发育图的进化图。它们就像一棵家谱，描绘了不同类型的有机体之间的关系。为螺旋虫构建这样一棵生命树将是关键的第一步，需要使用DNA序列、化石和活动物解剖学的信息。由此产生的进化树不仅仅是图表，因为它们允许研究进化是如何随着时间的推移进行的，使我能够估计过去进化变化的速度(速率)，以及发生的总变化量。这将使我了解从寒武纪到现在，新的形态变化是如何随着时间的推移而产生的。最后，它将让我了解寒武纪大爆发和随后的事件是如何塑造了今天仍然存在的壮观的生物多样性。

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.