Excavating the roots of the tree of life: revealing a billion year fossil record for the euglenids

在建立进化论的过程中，查尔斯达尔文意识到生命只起源一次，经过数十亿年的进化，变成了今天地球上令人眼花缭乱的多样性。自从这一巨大的范式转变以来，生物学的一个主要目标就是根据不同类型有机体的进化关系及其分歧的时间来建立“真正的生命树”。生命之树中最有问题和最不为人所知的区域是它的深层根：生命的起源和早期的多样化。这是因为这些事件发生在数十亿年前的远古时代，并且：（i）主要分歧为生命的三个领域（细菌，古生菌，真核生物）涉及一个复杂的有机体结合的'内共生事件';（ii）所涉及的生物是陌生的，因为现代的亲戚，如果有的话，已经发生了巨大的变化，随着时间的推移;（iii）化石记录是穷人在岩石从这样古老的时代;（iv）分子钟分析等技术随着时间的推移而变得不可靠。裸藻是当今地球上常见的一组奇怪的单细胞生物。它们栖息在淡水环境中，在水中使用一种称为“蠕动运动”的独特运动。有趣的是，它们要么以无机物为食（如动物），要么通过收集太阳的能量（如植物）。据信，他们可以做到后者，因为他们结合了光合作用的单细胞绿色藻类在“二次内共生事件”。我们许多人都很熟悉裸藻，因为它们在初级实验室课程中进行常规检查，以使学生熟悉单细胞真核生物的基本特征以及一些显示动物和植物特征的事实。裸藻特别迷人，因为对它们的解剖学和基因组的研究表明它们是最原始的最早的真核生物之一（即具有真正细胞的生物，并通过缺乏真正细胞的更基本的生物（细菌和古生菌）的组合进化而来）。不幸的是，眼虫缺乏可识别的化石记录，因此我们对它们的起源和进化历史知之甚少。为了解决这个主要问题，我们查阅了文献，发现了许多具有类似眼虫特征的化石。我们的洞察力是，我们已经发现了一种识别化石是否确实代表真正的裸藻的方法。现代裸藻具有独特的细胞壁结构，通过取其细胞壁的极薄切片（厚度小于1/10，000 mm）并在强大的透射电子显微镜下进行检查，可以识别这种独特的结构。我们已经对可能的裸藻化石进行了初步研究，并证明我们可以在化石中观察到这种结构，从而证明它们确实是裸藻。我们的建议是分析整个地质柱中潜在的裸藻化石，并通过展示具有裸藻壁结构特征的裸藻化石，为裸藻提供连续的化石记录。这将使裸藻科成为少数几个具有深层化石记录的早期分歧真核生物群之一（也是第一个超级群发掘）。这很重要，因为它将为最早的真核生物多样化的时间和性质提供证据。它还将为从事分子钟研究的分子生物学家提供一个重要的化石校准点。此外，我们正在解决一个高度热门的研究领域，我们的研究结果将燃料目前的争议，是否真核生物在海洋或淡水中进化，以及如何以及何时裸藻获得其次级内共生绿色藻。

In establishing the theory of evolution Charles Darwin realized that life originated only once and over billions of years diversified, through evolution, into the bewildering diversity of life on Earth today. Since this monumental paradigm shift a major goal of biology has been to establish the 'true tree of life' in terms of evolutionary relationships of the different types of organism and timing of their divergence. The most problematic and least understood regions of the tree of life are its deep roots: the origin of life and its early diversification. This is because these events occurred billions of years ago in the deep past and: (i) the primary divergence into the three domains of life (bacteria, archaea, eukaryotes) involved a complicated combining of organisms in 'endosymbiotic events'; (ii) the organisms involved are unfamiliar because modern relatives, if any, have changed dramatically through time; (iii) the fossil record is poor in rocks from such ancient times; (iv) techniques such as molecular clock analyses become unreliable the further back in time one investigates.The euglenids are a bizarre group of single-celled organisms common on the planet today. They inhabit freshwater environments where they move through the water using a unique motion called 'peristaltic movement'. Intriguingly, they either feed by ingesting matter (like animals) or through harvesting the Sun's energy (like plants). It is believed that they can do the latter because they combined with a photosynthetic unicellular green algae during a 'secondary endosymbiotic event'. Euglenoids are familiar to many of us as they are routinely examined in elementary laboratory classes, to familiarise students with the basic features of single-celled eukaryotes and the fact that some display characteristics of both animals and plants. Euglenids are particularly fascinating because studies of their anatomy and genome suggest they are among the most primitive of the earliest eukaryote organisms (that is organisms that have a true cell and evolved through the combination of more basic organisms (bacteria and archaea) that lack a true cell). Unfortunately euglenids lack a recognisable fossil record so we know little regarding their origin and evolutionary history.In order to remedy this major problem we have trawled the literature and discovered a number of fossils that have euglenid-like characters. Our insight is that we have discovered a way of recognising whether a fossil does indeed represent a true euglenid. Modern euglenids have a unique cell wall structure, and by taking extremely thin sections of their cell walls (less that 1/10,000 mm in thickness) and examining them under a powerful Transmission Electron Microscope, it is possible to identify this unique structure. We have undertaken preliminary studies on potential fossil euglenids and demonstrated that we can observe such structure in the fossils and hence prove that they are indeed euglenids. Some of the euglenid-like fossils are a staggering 1 billion years old.Our proposal is to analyse potential euglenid fossils from throughout the geological column and, by demonstrating which possess the characteristic euglenid wall structure, provide a continuous fossil record for the euglenids. This will place euglenids as one of the few groups of early divergent eukaryotes with a deep fossil record (and the first of the SuperGroup Excavates). This is important because it will provide evidence for the timing and nature of the diversification of the earliest eukaryotes. It will also provide an important fossil calibration point for molecular biologists that undertake molecular clock studies. Furthermore, we are addressing a highly topical research area and our findings will fuel current controversies concerning whether the eukaryotes evolved in the ocean or in fresh water and how and when euglenids acquired their secondary endosymbiotic green alga.