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

现存动物的多样性可以被分为大约30组，称为门，这个术语指的是拥有独特的身体计划。所有这些身体形态首次出现在化石记录中是在地质学的一眨眼之间，在一个被命名为寒武纪大爆发的事件中（大约5.5 - 5.2亿年前）。在这段时间里，各种各样的动物，如蠕虫、水母，甚至我们的远亲，都可以作为化石被发现，与之形成鲜明对比的是，更古老的岩石中，大多数生命的证据都是微观的。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一个“实验”阶段，动物的进化形式再也没有出现过。寒武纪大爆发被认为是地球历史上一个完全独特的事件，与其他任何时期相比，进化变化发生得更快，规模更大。不同体型的范围被称为差异，之前的一些研究提出，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进化的前身，填补了现存生物群体身体结构之间的空白。然而，对寒武纪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、蜈蚣和它们的亲戚），或者只考虑寒武纪生物和今天活着的生物的证据，而忽略了地球历史中5亿年的证据。自寒武纪大爆发结束以来，众所周知，动物们从根本上改变了它们的身体结构，通常是在迁移到新的、有时是极端和恶劣的环境时，比如从海洋向陆地生命的过渡，或者热液喷口的殖民。在这个项目中，我将使用一组被称为螺旋虫的动物来研究动物形态的多样性是如何从它们的起源寒武纪大爆发到现在形成的。螺旋藻种类繁多，既包括我们熟悉的蚯蚓和蜗牛，也包括一系列主要生活在海洋中的不寻常物种。螺旋体占动物身体结构的一半以上，从寒武纪大爆发开始的早期阶段就有很好的化石记录。综上所述，这些事实突出了这组生物作为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：在5.4亿年的螺旋动物进化过程中，形态空间占据模式（差异）是如何变化的？沿着谱系的什么进化模式导致了这些形态差异的模式？为此，我将从保存完好的寒武纪和奥陶纪早期（约5.42 - 4.8亿年前）的古化石中收集新数据，并将这些数据与现存物种的数据结合起来，构建称为系统发生的进化图。它们就像一个家谱，描绘了不同类型生物之间的关系。为螺旋体构建这样的生命树将是关键的第一步，这将需要使用来自DNA序列、化石和活体动物解剖结构的信息。由此产生的进化树不仅仅是图表，因为它们允许调查进化是如何通过时间进行的，允许我估计过去进化变化的速度（“速率”），以及发生的变化的总量。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今天壮观的生物多样性的。

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.