“A handful of scientists have become crusaders for the Gaia hypothesis, while the rest have dismissed it without a second thought. Toby Tyrrell, on the other hand, is one of the very few scientists to have considered the evidence at length and in detail. In summarizing nearly forty years of arguments for and against the Gaia hypothesis, he has done a great service for anyone who is curious about Gaia, or about this fascinating planet that we all call home.”–James Kirchner, University of California, Berkeley

“Toby Tyrrell unravels the various formulations of Gaia and explains how recent scientific developments bring the hypothesis into question. His criticisms are insightful, profound, and convincing, but fair. On Gaia is wonderfully informative and a pleasure to read.”–Francisco J. Ayala, author of Am I a Monkey?: Six Big Questions about Evolution

“At last, a beautifully written and clear-eyed analysis of the interplay of life and the Earth system. On Gaia provides the understanding for moving forward in the quest for sustainability, and is essential reading if our planet is to remain habitable for humanity.”–Thomas E. Lovejoy, George Mason University

“On Gaia makes a wonderful addition to the literature. It is scholarly, well-written, and well-reasoned.”–Simon A. Levin, Princeton University

Toby Tyrrell is professor of Earth system science at the National Oceanography Centre Southampton (University of Southampton).

ON GAIA

A CRITICAL INVESTIGATION OF THE RELATIONSHIP BETWEEN LIFE AND EARTH

By Toby Tyrrell

PRINCETON UNIVERSITY PRESS

Copyright © 2013 Princeton University Press
All rights reserved.
ISBN: 978-0-691-12158-1

Contents

Preface…………………………………………………………..ix1. Gaia, the Grand idea…………………………………………….12. Good Citizens or Selfish Genes?…………………………………..143. Life at the Edge: Lessons from Extremophiles……………………….474. Temperature Paces Life…………………………………………..675. Icehouse Earth………………………………………………….886. Given Enough Time ………………………………………………1137. Evolutionary innovations and Environmental Change…………………..1308. A Stable or an Unstable World?……………………………………1459. The Puzzle of Life’s Long Persistence……………………………..17110. Conclusions……………………………………………………199Notes…………………………………………………………….219Further Reading……………………………………………………273References………………………………………………………..277Acknowledgments……………………………………………………299Index…………………………………………………………….301

Excerpt

CHAPTER 1

GAIA, THE GRAND IDEA

This first chapter introduces the Gaia hypothesis and two competinghypotheses.

1.1. A Brief History

Gaia, the idea that life moderates the global environment to make it more favorablefor life, was first introduced in 1972 in an academic paper titled “Gaiaas Seen through the Atmosphere” in the journal Atmospheric Environment, followedrapidly by two other papers both in 1974: “Atmospheric Homeostasis byand for the Biosphere” in the journal Tellus, and “Biological Modulation of theEarth’s Atmosphere” in the journal Icarus. James Lovelock was sole author ofthe first paper and coauthor with Lynn Margulis of the latter two. Both were alreadyscientists of some note. Lovelock had already pursued a successful careerinventing chemical instruments, including, most famously, the electron capturedetector. This device, when coupled to a gas chromatograph, allows for thedetection of trace chemical substances even at extremely low concentrations.Before that, Lovelock had worked for twenty years at the United Kingdom’sNational Institute for Medical Research in Mill Hill, London, carrying out researchin biomedical science.

Use of the electron capture detector started to become widespread due to itsgreat utility, and through his consultancy work with it Lovelock was invited toparticipate in a NASA project to work out how to ascertain if Mars containedlife. The two Viking spacecraft, now revered in history as the first spacecraftever to land on the surface of another planet, were just then being designed, anda major priority was to decide which instruments to put on board. Reflectionon this problem of how to detect the presence of life stimulated Lovelock’s firstthoughts on the Gaia hypothesis.

Lynn Margulis was a groundbreaking microbiologist at Boston University.She had long been championing her own (separate) revolutionary idea, onethat is now widely accepted. It proposed that in the evolutionary distant pastone primitive cell managed to “enslave” another (engulf it without killing it)and in the process benefited from the new capabilities of the enslaved cell. Sheproposed that such “endosymbiosis” had occurred a number of times. Mitochondria,chloroplasts, and flagella are all part of the machinery of individualcells; they are subcomponents of many single-celled creatures and of individualcells in multicellular organisms. According to the endosymbiosis theory theyare all suggested to be relics of long-ago-assimilated single cells. Each cell ofevery animal and plant, including those making up human bodies, is from thisperspective seen as an evolutionary amalgam of several different ancient lineages.This theory, initially treated with some considerable skepticism (an earlyMargulis paper on it was rejected by as many as fifteen different scientific journalsbefore being accepted), is now the consensus view. Although she was notthe first to conceive of the idea, Margulis was the first to support it with directmicrobiological observations, and it was in large part thanks to her continuedchampioning of it, against strong opposition, that it came to be widely accepted.It has considerable implications. For example, it requires some modificationof the idea that evolution proceeds solely by selection among organisms, eachof which is a slightly modified descendant of the previous generation. Amongthe unicellular microbes at least, evolution has at times created a radically newspecies in a single jump, as a novel intracellular symbiont has been acquired.

Although Margulis jointly authored some of the early papers and remained achampion of Gaia, the hypothesis has always been first and foremost the brainchildof James Lovelock. Following the Lovelock and Margulis papers and someother papers in academic journals, none of which generated large amounts ofinterest or attention, Lovelock brought out a book called Gaia: A New Look atLife on Earth. When this book came out, in 1979, it brought Gaia to scientificprominence at last. The book stimulated a mixture of admiration and oppositionamong scientists. Many evolutionary biologists, in particular, were verycritical, for reasons that will be explained in the next chapter. Some of the biologists’objections were subsequently countered by modifying the hypothesis andalso by the production of a now-famous model, Daisyworld. This model demonstratedthe theoretical possibility of stable regulation of planetary temperatureby organisms that are still adhering to biologically plausible rules of behavior andreproduction. The Daisyworld paper, by Andrew Watson and James Lovelock,came out in 1983 in the academic journal Tellus. In the ensuing years Lovelockproduced two more books: The Ages of Gaia: A Biography of Our Living Earth, in1988, and Gaia: The Practical science of Planetary Medicine, in 1991.

In the 1980s the Gaia hypothesis was considered both interesting and controversialand continued to attract a mixture of agreement, interest, doubt, and rejection.By the mid-1980s it was decided that there was sufficient interest to meritorganizing an international conference. In 1988 a prestigious Chapman Conferenceof the American Geophysical Union brought together advocates and interestedskeptics in a wide-ranging scientific discussion of the hypothesis. Furtherinternational conferences on Gaia were convened at Oxford University in 1994,1996, and 1999, with membership primarily by invitation. In 2000 a second openScientists on Gaia Chapman Conference was held in Valencia, Spain.

As Lovelock, now in his nineties, has become less active, others have takenup the torch. Tim Lenton, for instance, an Earth system scientist at the Universityof Exeter, has written many papers on Gaia, including a review article inNature in 1998. The Gaia hypothesis had achieved a degree of scientific respectability.However, a brief review of its reception in books published since 2007shows that while it is now accepted gladly by some, it also continues to stimulateintense debate. This was also revealed in back-and-forth exchanges in thepages of the journal Climatic Change in 2002 and 2003. Nevertheless, wheninterviewed for a biography published in 2009, Lovelock claimed that Gaia hasmade the transition from being just a hypothesis to being solid science.

The degree to which Gaia has been accepted by a large part of the scientificcommunity, including those in its higher echelons, was highlighted by theAmsterdam Declaration on Global Change. This document is a synthesis ofthe work of four international research umbrella organizations, including theInternational Geosphere-Biosphere Programme (IGBP) and the World ClimateResearch Programme (WCRP) and was discussed at a conference attended bymore than one thousand scientific delegates. The second paragraph of the declarationasserts: “Research carried out over the past decade under the auspicesof the four programmes to address these concerns has shown that: The EarthSystem behaves as a single, self-regulating system comprised of physical, chemical,biological and human components.” The wording could almost have beenlifted from one of Lovelock’s books. A Nature editor, reporting on the secondChapman Conference in 2000, judged that “James Lovelock’s theory of the bioticregulation of Earth has now emerged with some respectability followingclose scrutiny by the biogeochemical community.”

Is the scientific respectability and the continuing prominence justified? Readthe rest of this book if you want to find out.

1.2. The Hypothesis

The Gaia hypothesis is nothing if not daring and provocative. It proposes planetaryregulation by and for the biota, where the “biota” is the collection of all life.It suggests that life has conspired in the regulation of the global environment soas to keep conditions comfortable. During the more than two (probably morethan three) billion years that life has existed as a continuous presence on Earth,Lovelock suggests that life has had a hand on the tiller of environmental control.And the intervention of life in the regulation of the planet has been such asto promote stability and keep conditions favorable for life.

That, in a nutshell, is the hypothesis. Providing a more precise definition is,however, made difficult by a couple of factors: (1) the hypothesis has not stayedconstant but instead has been modified over time in response to criticisms;and (2) Lovelock’s publications do not provide a completely clear definition, althoughothers have tried subsequently to clarify it for him, as described below.

Gaia is not a hard-and-fast, well-defined concept. It is not a “set menu.”Rather it is more like a loosely defined smörgåsbord, from which “diners” cantake their pick from a collection of several related hypotheses, often couched inrather vague terms. The lack of clarity presents a problem for those of us whowant to analyze and evaluate Gaia. It may even seem a poor basis for a booksuch as this one. However, fortunately, there are central components of Gaiathat are fundamental to all definitions, and it is these that I examine in thisbook. These concepts are at the heart of the hypothesis and are present regardlessof which variant is chosen:

A. Earth is a favorable habitat for life.

B. It has been so over geologic time as the environment has remained fairly stable.

C. This is partly due to life’s role in shaping the environment. For instance, life hasinfluenced the chemical composition of the atmosphere and the sea.

In Lovelock’s own words, the hypothesis has been defined in various differentways over the years:

We have since defined Gaia as a complex entity involving the Earth’s biosphere,atmosphere, oceans and soil; the totality constituting a feedback or cyberneticsystem which seeks an optimal physical and chemical environment for life on thisplanet. The maintenance of relatively constant conditions by active control maybe conveniently described by the term “homeostasis.” (Lovelock 1979)

The main part of the book … is about a new theory of evolution, one thatdoes not deny Darwin’s great vision but adds to it by observing that the evolutionof the species of organisms is not independent of the evolution of their materialenvironment. Indeed the species and their environment are tightly coupled andevolve as a single system. What I shall be describing is the evolution of the largestliving organism, Gaia. (Lovelock 1988)

The concept that the Earth is actively maintained and regulated by life on itssurface. (Ibid.)

Gaia theory predicts that the climate and chemical composition of the Earthare kept in homeostasis for long periods until some internal contradiction or externalforce causes a jump to a new stable state. (Ibid.)

Gaia is the Earth seen as a single physiological system, an entity that is alive atleast to the extent that, like other living organisms, its chemistry and temperatureare self-regulated at a state favourable for life. (Lovelock 1991)

The top-down view of the Earth as a single system, one that I call Gaia, isessentially physiological. It is concerned with the working of the whole system,not with the separated parts of a planet divided arbitrarily into the biosphere, theatmosphere, the lithosphere, and the hydrosphere. (Ibid.)

Organisms and their environment evolve as a single, self-regulating system.(Lovelock 2003b)

The hypothesis that living organisms regulate the atmosphere in their owninterest. (Ibid.)

By the end of the 1980s there was sufficient evidence, models and mechanisms,to justify a provisional Gaia theory. Briefly, it states that organisms and their materialenvironment evolve as a single coupled system, from which emerges thesustained self-regulation of climate and chemistry at a habitable state for whateveris the current biota. (Ibid.)

In Gaia theory, organisms change their material environment as well as adaptto it. (Ibid.)

And from Tim Lenton:

The Gaia theory proposes that organisms contribute to self-regulating feedbackmechanisms that have kept the Earth’s surface environment stable and habitablefor life. (Lenton 1998)

Some changes have been made to the hypothesis over time. A first correctionwas to alter the proposed life effect on the environment, from one of makingit optimal to one of making it comfortable:

The first edition of this book used the terms optimum and optimize too freely;Gaia does not optimize the environment for life. I should have said that it keepsthe environment constant and close to a state comfortable for life. (Lovelock1979, in the preface to a 1987 revised edition)

A second clarification was to renounce any ascribing of purpose or intent tothe biota. It was made clear that any biotic regulation of the environment mustbe automatic and unconscious. The reason for their impacts on the environmentis not because the organisms responsible consciously want to help outtheir brothers in life:

At first we explained the Gaia hypothesis in words such as “Life, or the biosphere,regulates or maintains the climate and the atmospheric composition at an optimumfor itself.” This definition was imprecise, it is true; but neither Lynn Margulisnor I have ever proposed that the planetary regulation is purposeful. (Lovelock1991)

This topic is returned to in the next chapter.

An attempt to seek greater clarity of definition came from James Kirchner,an Earth scientist at the University of California, Berkeley. The first Scientistson Gaia conference, in 1988, was the first time that large numbers of bothproponents and interested skeptics got together in open debate. As might beexpected, there were some surprises. One novel contribution was a paper byJames Kirchner titled “The Gaia Hypotheses: Are They Testable? Are They Useful?”This paper revealed significant differences between the definitions ofGaia in the various papers and books and attempted to classify them. Kirchnerproposed the following taxonomy of hypotheses, on a gradient from weakly tostrongly controversial: (1) influential Gaia, which asserts only that biology affectsthe physical and chemical environment to some degree; (2) coevolutionaryGaia, which limits itself to stating that the biota and environment are somehowcoupled; (3) homeostatic Gaia, which emphasizes the stabilizing effect of thebiota; (4) teleological Gaia, which implies that the biosphere is a contrivancespecifically arranged for the benefit of the biota; and (5) optimizing Gaia, whichsuggests that the biosphere is optimized in favor of the biota.

Kirchner’s paper was perhaps unreasonable in one regard, in that the twoweakest forms were weaker than adopted in any major Gaian publication.Nevertheless, the paper made a vital contribution by pointing to ambiguities inhow Gaia was defined. It was an important influence on thinking at that time.Lovelock later retreated from the two stronger forms of Gaia. Even with regardto homeostatic Gaia, Kirchner called for greater clarity in terms of thedefinition: “What is stability? Does it mean resistance to change, resilienceunder change, or bounds on the magnitude of change?” Stability is dealt within greater detail in chapter 8.

1.3. Supporting Evidence

It is one thing to form a hypothesis, but something else entirely to show thatthe hypothesis should be taken seriously. Anyone can formulate a hypothesis,in seconds. Demonstrating that it is a true picture of reality is quite anothermatter. Why should we believe the Gaia hypothesis? What reasons are there tosuggest that it corresponds to nature, or, in other words, that it accords with thereality of how the Earth’s environment has been regulated and maintained? Inhis books and papers, Lovelock has presented a wide variety of evidence. Thereare three main facts, or classes of facts, that he has advanced in support of thehypothesis. These three assertions are carefully examined in this book:

Assertion No. 1: the environment is very well suited to the organisms thatinhabit it.

In the first and subsequent Gaia books, the tolerances of organisms for temperature,pH, salinity, and other environmental parameters were described.The fact that the Earth environment satisfies these tolerances (is habitable) isput forward as strong evidence for Gaia. This assertion is critically analyzed inchapters 3, 4, and 5 of this book.

Assertion No. 2: the Earth’s atmosphere is a biological construct whose compositionis far from expectations of (abiotic) chemical equilibrium.
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