Major transitions in evolution: understanding the fossil evidence

地球上生命的进化经历了一系列重大的转变，遗传信息的组织和世代之间的传递方式发生了变化。这些转变涉及选择水平的变化。例如，在从单细胞生物向多细胞生物转变的过程中，选择从作用于单个细胞转变为作用于一组细胞。这种转变带来了新的机会，在这种情况下，包括细胞之间分工的可能性，或某些细胞专门从事特定任务。因此，主要的转变使生物有可能进化到更高的复杂程度。我的研究重点是发生在前寒武纪的重要的早期进化转变。这些包括区室化细胞的起源，真核生物（有核生物）的起源，多细胞生物的起源和动物的起源。我将重新评估这些转变的化石证据，并测试之前提出的驱动它们的机制。这对于理解早期地球上生物与环境之间的关系至关重要。它还将为科学家们试图确定其他行星上生命的证据提供重要信息。直接研究这些早期过渡的唯一方法是通过前寒武纪化石记录。一些前寒武纪化石被认为对理解这些转变很重要，但它们经常引起争议，有关它们的说法经常受到挑战。一个关键问题是，许多诊断结构相对简单。因此，很难区分已经被风化的真正生物特征和由风化期间和之后发生的衰变或晶体生长产生的伪影。我的研究旨在通过使用两种互补的方法将真正的生物学与人工制品区分开来，这将使我们能够对前寒武纪化石记录进行急需的重新评估。首先，一系列生物体以及被认为在生命起源中发挥作用的非生物结构将在实验条件下衰变。这将使我能够确定诸如细胞核和其他亚细胞结构的结构是否可以可行地保存下来。它还将使我能够确定腐烂对有机壁特征的影响，这些特征对识别真核生物化石和动物休眠阶段至关重要。我还将测试脂质囊泡的保存潜力，脂质囊泡可以在没有生命的情况下产生，并被认为在细胞的起源中发挥作用。然而，如果保存在岩石记录中，脂质囊泡也有可能被误认为是真正的细胞。这项研究将确定脂质囊泡的保存是否可行，这将对了解最早的化石记录以及来自其他行星的潜在生命具有重要意义。其次，一系列前沿分析将应用于前寒武纪化石。其目的是确定（a）保存的生物组织和（B）后期矿物生长造成的人工制品的质地和化学性质。一旦在没有争议的化石中确定了每种矿物生长方式的特征，这些信息就可以用来解释更有争议的材料。这两条证据的发现将被用来对前寒武纪化石记录进行批判性的重新评估。这种重新评估将提高我们对早期进化中主要转变的理解。这些信息将用于分析，以测试主要过渡的拟议驱动机制。例如，有人提出，在许多组中，增加的氧气水平促使多细胞结构的转变。分析将使用新的技术，使这样的建议得到测试。

The evolution of life on Earth has seen a series of major transitions in the way genetic information is organised and transmitted between generations. These transitions involve shifts in the level of selection. For example, in the case of the transition from single-celled to multicellular organisms selection shifted from acting on single cells to acting on a group of cells. Such shifts opened up new opportunities that, in this case, include the possibility of division of labour between cells, or the specialization of certain cells for specific tasks. As a result the major transitions make it possible for organisms with greater degrees of complexity to evolve.My research focuses on the important early evolutionary transitions that took place in the Precambrian. These include the origin of compartmentalized cells, the origin of eukaryotes (organisms with nuclei), the origin of multicellular organisms and the origin of animals. I will both reappraise the fossil evidence for the transitions and also test previous suggestions of mechanisms responsible for driving them. This will be critical for understanding the relationship between biology and environment on the early Earth. It will also provide important information for scientists attempting to identify evidence of life on other planets.The only way that these early transitions can be directly studied is through the Precambrian fossil record. A number of Precambrian fossils have been proposed as being important in understanding these transitions, but they have often been controversial and the claims made about them have frequently been challenged. A key problem is that many of the diagnostic structures are relatively simple. As a result, it can be difficult to distinguish between genuine biological features that have been fossilized and artefacts that result either from decay or from crystal growth that occurs during and after fossilization. My research aims to tease apart genuine biology from artefact by using two complimentary approaches that will enable a much-needed reappraisal of the Precambrian fossil record. Firstly, a range of organisms, as well as non-biological structures that have been suggested to have a role in the origin of life, will be decayed under experimental conditions. This will enable me to determine whether structures such as nuclei and other subcellular structures can feasibly be preserved. It will also enable me to determine the effects of decay on features of organic walls that are vital in identifying fossil eukaryotes and animal resting stages. I will also test the preservation potential of lipid vesicles, which can be generated in the absence of life and have been proposed to play a role in the origin of cells. However, if preserved in the rock record, lipid vesicles would also have the potential to be mistaken for true cells. This research will determine whether the preservation of lipid vesicles is feasible, which will have important implications for understanding the earliest fossil record as well as potential life from other planets.Secondly, a range of cutting edge analyses will be applied to Precambrian fossils. These aim to characterize the texture and chemistry of (a) preserved biological tissue; and (b) artefacts caused by later mineral growth. Once the characteristics of each style of mineral growth have been established in uncontroversial fossils, the information can be used to interpret more contentious material.The findings from these two lines of evidence will be used to critically reappraise the Precambrian fossil record. This reappraisal will improve our understanding of major transitions in early evolution. This information will be used in an analysis to test proposed driving mechanisms for major transitions. For example, it has been suggested that increased oxygen levels drove switches to multicellularity in a number of groups. The analysis will use new techniques that enable proposals such as this to be tested.