Revolutions that made the Earth

lenton Rating: ★★★★☆

Tim Lenton and Andrew Watson. 2013. Revolutions that made the Earth. Oxford University Press. ISBN 978 0 19 967346 9.

One of the problems with geology teaching is that the size of the events and the timescales involved are often too great for the tyro to comprehend. Sometimes the problem is that there are just too many pieces to put together and what you really need is something that tells the big picture (in much the same way that plate tectonic theory made British stratigraphy so much easier to comprehend!). This book is formed on the same principles – tell a really big story and show how all the parts are connected. As much as we experiment with this planet, perhaps we should be aware of just how “amazing” it is and how unlikely that we actually made it here. It’s the aim of this book to chart our unlikely situation and how it might all change if we don’t heed some of the big lessons from the past.

The central thesis is that the Earth has gone through a series of revolutions (or at least significant changes, with recurring patterns that suggest revolutions!) to get to our current conditions. All of these have been part of the Earth system whereas, now, we seem to be making our own revolution not truly knowing how it will end (much along the lines, it seems of discussions about the “Anthropocene“).

We start with four chapters focussed on the very early Earth. The first point is the very beginning – the earliest rocks and how they can be identified. There is some mention of the possible life forms to be found over 3 billion years ago. It’s a vast time but with little obviously happening in terms of revolutions, we move on to the edge of the Proterozoic where we can see the first signs of an oxygen atmosphere and the rise and fall of at least two episodes of total ice coverage (the so-called ‘snowball Earth‘ concept). The final chapter in this part examines the idea of  ‘revolutions’, the first time we get to see the thesis laid out clearly. In essence, there are four – inception (formation of the planet), oxygen, complexity (in both biotic and abiotic areas) and “us”. Part two looks at two, largely competing, models of life on Earth. The first takes our own physical needs and tries to consider how they might arise whilst the second starts off with an Earth and then considers what feedbacks might achieve. A final chapter delves at some length into the Gaia hypothesis and explores what these ideas mean for our developing planet.

Part 3 moves the text into new directions, a more technical analysis because it looks at topics, not time. Part three starts with what you could either say was the a great revolution or a mass poisoning depending on whether you needed oxygen for survival or actively avoided it. It’s the time, in the late Archaen/early Proterozoic (or about 2.3 billion years ago), when the rise of oxygen became unstoppable. True, it was only at low concentrations compared to today but it did signal one of the more significant changes. Three chapters examine this phenomenon through a study of the evolution of photosynthesis,. It moves on to look at the development of earlier, less successful times when oxygen appeared. Finally, we get to examine the main event, the Great Oxidation, caused, it appears, by a rise of ozone concentrations and the escape of hydrogen. Whatever the exact mechanism (and the explanation here is backed, like the rest of the text, with copious references) it was the final coming together of a set of factors that allowed it to develop and expand. After this revolution, the world would literally never be the same again.

Part four takes up the story of the implications of  oxygen – namely the rise of more complex cells. Whilst it is interesting to speculate if life would have evolved to the extent it has without oxygen, it cannot be denied that when the levels did rise sufficiently, there was an “explosion” of cellular diversity. The start was the development of the eukaryotes from the prokaryotes.  It used to be considered that these two were completely distinct, but now we find examples that cross the divide.  Whilst there is debate about how the development took place, we are also wondering about the when of this event. There are numerous possibilities, most stretching back to about 2.4 bn years ago. As we contemplate the correct age of appearance we also find ourselves wondering about what sort of evidence actually characterises multi-cellular life! Finally, in this section, we see the rise of life in the pre-Cambrian – originally a time considered too old for life which has, in the last 30 years, been part of the most remarkable turnaround in geological thought. Rather than have a Cambrian “explosion” out of no-where, we see more of a Cambrian ‘radiation’.

Part five takes the story one stage further and considers the development of life from the Cambrian onwards. We start with that explosion and the impact of the still-developing atmosphere on its evolution.  From there, the next major development is the spread of species to land and the physiological impact that had. Plants colonised the land and were more efficient energy producers. They also had the ability to re-cycle nutrients and s0 now we had a system of interdependence growing between biotic and abiotic environments. Finally, there’s an analysis of the great extinctions in this time – periods when a vast majority of species became extinct.

The final section looks at the development of the human species and the environmental conditions that were seen at the time. One of our biggest issues is the extent to which human development was helped or hindered by climatic change. We have had both hot periods and Ice Ages and yet our current climatic conditions seem the most benign for millennia. It is tempting to suggest that we could not survive outside our current narrow range but we have done it before, not just in the numbers we have now! From climate to human physiological and social development. As we evolved into the species we see today we also developed social systems to enable us to continue. Thus Homo sapiens moved from small numbers of hunters to larger numbers of farmers. We have reached the current time. A very brief chapter reviewing the systems model of Earth dynamics leads the reader to a final consideration of where the planet might head.

This is a very interesting book. It is a completely new take on the geological development of the Earth and the impacts arising. It looks less at the temporal side and more on the idea that certain “revolutions” have pushed along  what might well have been a sideline in planetary development. By taking the perspective of the change rather than the time when it took place, the authors have lifted the text out of the introductory-geology range and into a genuinely new appreciation of how we can understand events in Earth history. Starting with basic chemistry and the divisions of life, the reader is taken through theories of Earth science and a series of evolutions (or maybe the revolutions of the title) – life, complexity and diversity, finishing with a reasoned argument about where we might be headed. It’s a familiar story but one told in a unique way. It’s not always an easy read because it requires considerable scientific understanding but it is a rewarding one. It’s a great antidote to the more rigid, temporal examination of the Geological Record and one that deserves a wide readership.

 

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