Study Guide for Evolving Life and the EarthEvolving Life and the Earth This book explores the history of interactions between life, as it has evolved, and the Earth, with particular emphasis on the impact of eukaryotes. You might like to think of it as life’s eye-view of ‘Earth and Life’. A pair of complementary Video Bands, ‘The Cretaceous Greenhouse: a surfeit of carbon’ and ‘The Cretaceous Greenhouse World: poles apart’, are particularly relevant to this book. The book is assessed in TMA 04. (Note that there are self-assessment questions in the Multiple-choice questions and answers booklet. You should complete these as you study the text and they will also be useful for revision of the Course.) Chapter 1 The evolution of evolution The first chapter opens with an overview of the major milestones in the history of life on Earth, as encountered earlier in the Course. It then focuses on some of the innovations in cell structure and function (chromosomes, internal protein skeleton, sexual reproduction) that conferred remarkable evolutionary exuberance on the eukaryotes, and allowed the development of multi-tiered trophic pyramids. We go on to consider how evolution by natural selection adapts organisms to the circumstances in which they live, and who or what might be said to benefit from the process. Next we delve into the distant past, looking at the evidence for the origin (at least 2100 Ma ago) and early history of the eukaryotes, involving multiple endosymbioses. The emergence of complex marine ecosystems through the Proterozoic is traced, culminating in the first major radiation of animals, less than 600 Ma ago in the Vendian. Chapter 2 Feedbacks between the late Proterozoic Earth and life Here, we take a closer look at the interactions between the late Proterozoic Earth and life. Geological and geochemical evidence (especially the record of changes in marine carbon and strontium isotope ratios) suggest that large-scale burial of organic matter occasionally promoted glaciations, while hydrothermal effusions damped the rise of oxygen levels until Vendian times – suggesting a possible trigger for the rise of animals. Chapter 3 Life’s ups and downs in the Phanerozoic The Vendian fauna was rapidly replaced at the start of the Cambrian by more complex animals, many with skeletal hard parts. This chapter investigates how life habits became more diversified, including a sharp rise in predation, and how these changes in their turn influenced biogeochemical cycles. It then illustrates how the abundant fossil record inaugurated by the spread of skeletal hard parts testifies to a series of major periods of diversification, punctuated by mass extinctions evidently linked with a variety of environmental crises. These biotic changes had yet further feedback effects. Chapter 4 Greening of the land One of the most important evolutionary innovations, explored in this chapter, was the rise of land plants, which added a new dimension to life’s relationship with the Earth: land vegetation now accounts for most of the Earth’s non-bacterial biomass, and significantly modulates climate. Feedbacks in early land plant evolution (affecting water supply, access to light, gas exchange, support, reproduction, and even the Earth’s albedo) are investigated. Chapter 5 A closer look at climate The interplay between the Earth’s geography and internal dynamics, and Phanerozoic life, is analysed in the next three chapters. General controls on climate, in particular, are outlined in this chapter, leading on to a look at climate modelling. Chapter 6 An icehouse case study Two case studies follow: this first one charts the Earth’s passage through a cold, ‘icehouse’ period in the late Carboniferous and early Permian, terminated by climatic warming in the late Permian. Again, a combination of geological and geochemical evidence implicates major changes in the global carbon cycle, driven by the interactions between geography and life, as the key to changes in atmospheric composition and hence climate. A complex combination of factors is found to have contributed to the largest recorded mass extinction at the end of the Permian. Chapter 7 A greenhouse case study This second case study depicts a contrasting, ‘greenhouse’ period of widespread warmth in the Cretaceous. One difference between these two episodes highlighted here is the major shift of land plant primary production to higher latitudes with climatic warming. The spotlight is also put on another aspect of the Cretaceous greenhouse world – the vast increase in the rate of burial of carbon, especially in marine carbonates, implying a commensurately augmented supply, probably linked with superplume activity. Changes in mantle dynamics thus emerge as a key influence on climate modes, entailing major reorganizations of ecosystems. Chapter 8 The relationship between evolving life and the Earth The final chapter of the book draws together the conclusions of the previous chapters to address the fundamental question concerning life’s relationship with the Earth: benign partnership, or a chaotic system lurching from one temporary state of balance to another? The theoretical and empirical evidence reviewed here favours the latter condition, contrary to the expectations of the ‘Gaia hypothesis’. Required background and the most difficult sections Previous books in this Course will have provided most of the background knowledge and skills that you will need in order to study this book, though familiarity with Discovering Science (S103) or its predecessor (S102) will also be of considerable benefit, especially with respect to the more biological topics addressed. In particular, it will be useful to have some understanding of: •basic genetics and the theory of evolution by natural selection (for Chapter 1); •factors affecting the isotopic ratios of carbon and strontium recorded in marine limestones (for Chapters 2, 3, 6 and 7); •the use of simple quantitative models for investigating the effects of changes in conditions at the Earth’s surface (for Chapters 4, 5 and 7). Quite a lot of information is presented in this book, however it is there only to support the major principles being discussed, so it is important for you not to lose sight of the wood for the trees. Although much of Chapter 1 may seem much more purely biological in flavour than earlier parts of the Course, you will find that it provides an important perspective on the nature of the relationship between the Earth and life, complementing those derived from the previous books and the later chapters of this book. Chapter 8, though short, is crucial in this respect, as it draws these various perspectives together: be sure to read it carefully. Video programmes Two linked video programmes explore aspects of the Cretaceous greenhouse world. The first (Video Band 7, ‘The Cretaceous Greenhouse: a surfeit of carbon’) reviews the evidence for increased rates of carbon burial then, and its likely sourcing from superplume activity. The second (Video Band 8, ‘The Cretaceous Greenhouse World: poles apart’) focuses on the high latitude land flora of the time, to illustrate both the evidence it provides for climatic warmth and the character of an entire ecosystem that no longer exists in our modern icehouse world. Also relevant, to the concluding discussion in Chapter 8, is Video Band 6 (‘Daisyworld’), which you may wish to watch again when you study that chapter: note that the book and the video programme present contrasting views concerning the nature of the relationship between life and the Earth. Website Read the following sections on the S269 website: Evolving Life and the Earth – Topics Course Items and TMAs Other Resources Copyright © 2003 The Open University SUP 78371 1 Back to S269 |