Gaia hypothesis

The Gaia hypothesis, a hypothesis put forward to explain a number of paradoxes about life and the earth, was first formulated in the 1960s by the independent research scientist James Lovelock. Until 1975 it was almost totally ignored. An article in the New Scientist of February 15th, 1975, and a popular book length version of the theory, published as The Quest for Gaia, began to attract scientific and critical attention to the hypothesis. Championed by certain environmentalists and scientists, it was vociferously rejected by many others, both within scientific circles and outdie of them. .

The Gaia hypothesis forms part of what is scientifically referred to as earth system science, and is a class of scientific models of the geo-biosphere in which life as a whole fosters and maintains suitable conditions for itself by helping to create an environment on Earth suitable for its continuity. The first such theory was created by Lovelock, who was working with NASA when he developed his hypotheses in the 1960s. He wrote an article in the science journal Nature, before formally publishing the concept in the 1979 book Gaia: A new look at life on Earth. He hypothesized that the living matter of the planet functioned like a single organism and named this self-regulating living system after the Greek goddess Gaia, using a suggestion from the novelist William Golding.

Since 1971, the noted microbiologist Lynn Margulis has been Lovelock's most important collaborator in developing Gaian concepts (Turney 2003).

Gaia "theories" have non-technical predecessors in the ideas of several cultures. Today, "Gaia theory" is sometimes used among non-scientists to refer to hypotheses of a self-regulating Earth that are non-technical but take inspiration from scientific models. Among some scientists, "Gaia" carries connotations of lack of scientific rigor and quasi-mystical thinking about the planet Earth, and therefore Lovelock's hypothesis was received initially with much antagonism by much of the scientific community. No controversy exists, however, about the fact that living organisms are influenced by their physical environment and vice versa (see Ecology).

Range of views

The original Gaia hypothesis has split into a spectrum of hypotheses, ranging from the undeniable (Weak Gaia) to the radical (Strong Gaia).

At one end of this spectrum is the undeniable statement that the organisms on the Earth have radically altered its composition. A stronger position is that the Earth's biosphere effectively acts as if it is a self-organizing system, which works in such a way as to keep its systems in some kind of meta-equilibrium that is broadly conducive to life. The history of evolution, ecology and climate show that the exact characteristics of this equilibrium intermittently have undergone rapid changes, which are believed to have caused extinctions and felled civilisations (see climate change).

Biologists and earth scientists usually view the factors that stabilize the characteristics of a period as an undirected emergent property or entelechy of the system; as each individual species pursues its own self-interest, for example, their combined actions tend to have counterbalancing effects on environmental change. Opponents of this view sometimes point to examples of life's actions that have resulted in dramatic change rather than stable equilibrium, such as the conversion of the Earth's atmosphere from a reducing environment to an oxygen-rich one. However, proponents will point out that those atmospheric composition changes created an environment even more suitable to life.

Some go a step further and hypothesize that all lifeforms are part of one single living planetary being called Gaia. In this view, the atmosphere, the seas and the terrestrial crust would be results of interventions carried out by Gaia through the coevolving diversity of living organisms. While it is arguable that the Earth as a unit does not match the generally accepted biological criteria for life itself (Gaia has not yet reproduced, for instance; it still might spread to other planets through human space colonization and terraforming), many scientists would be comfortable characterising the earth as a single "system".

The most extreme form of Gaia hypothesis is that the entire Earth is a single unified organism; in this view the Earth's biosphere is consciously manipulating the climate in order to make conditions more conducive to life. Scientists contend that there is no evidence at all to support this last point of view, and it has come about because many people do not understand the concept of homeostasis. Many non-scientists instinctively see homeostasis as an activity that requires conscious control, although this is not so.

Much more speculative versions of Gaia hypothesis, including all versions in which it is held that the Earth is actually conscious or part of some universe-wide evolution, are currently held to be outside the bounds of science. These are discussed in the Gaia philosophy article. Also outside the bounds of science is the Gaia Movement, a collection of different organisations operating in different countries, but all sharing a concern for how humans might live more sustainably within the "living system".

Gaia Theories

Early modern parallels

In Lives of a Cell (1974), the biologist, Lewis Thomas, makes an observation very similar to Lovelock's Gaia hypothesis:

I have been trying to think of the earth as a kind of organism, but it is no go. I cannot think of it this way. It is too big, too complex, with too many working parts lacking visible connections. The other night, driving through a hilly, wooded part of southern New England, I wondered about this. If not like an organism, what is it like, what is it most like? Then, satisfactorily for that moment, it came to me: it is most like a single cell.

Lovelock initial hypothesis

Lovelock defined Gaia as:

a complex entity involving the Earth's biosphere, atmosphere, oceans, and soil; the totality constituting a feedback or cybernetic system which seeks an optimal physical and chemical environment for life on this planet.

His initial hypothesis was that the biomass modifies the conditions on the planet to make conditions on the planet more hospitable – the Gaia Hypothesis properly defined this "hospitality" as a full homeostasis. Lovelock's initial hypothesis, accused of being teleological by his critics, was that Gaia atmosphere is kept in homeostasis by and for the biosphere.

Lovelock suggested that life on Earth provides a cybernetic, homeostatic feedback system operated automatically and unconsciously by the biota, leading to broad stabilization of global temperature and chemical composition.

With his initial hypothesis, Lovelock claimed the existence of a global control system of surface temperature, atmosphere composition and ocean salinity. His arguments were:

• The global surface temperature of the Earth has remained constant, despite an increase in the energy provided by the Sun.
• Atmospheric composition remains constant, even though it should be unstable.
• Ocean salinity is constant.

Since life started on Earth, the energy provided by the Sun has increased by 25% to 30%; however the surface temperature of the planet has remained remarkably constant when measured on a global scale. Furthermore, he argued, the atmospheric composition of the Earth is constant. The Earth's atmosphere currently consists of 79% nitrogen, 20.7% oxygen and 0.03% carbon dioxide. Oxygen is the second most reactive element after fluorine, and should combine with gases and minerals of the Earth's atomosphere and crust. Traces of methane (at an amount of 100,000 tonnes produced per annum), should not exist, as methane is combustible in an oxygen atmosphere. This composition should be unstable, and its stability can only have been maintained with removal or production by living organisms.

Ocean salinity has been constant at about 3.4% for a very long time. Salinity stability is important as most cells require a rather constant salinity degree and do not tolerate much values above 5%. Ocean salinity constancy was a long-standing mystery, because river salts should have raised the ocean salinity much higher than observed. Recently it was suggested that salinity may also be strongly influenced by seawater circulation through hot basaltic rocks, and emerging as hot water vents on ocean spreading ridges. However, the composition of sea water is far from equilibrium, and it is difficult to explain this fact without the influence of organic processes.

The only significant natural source of atmospheric carbon dioxide (CO₂) is volcanic activity, while the only significant removal is through the precipitation of carbonate rocks. In water, CO₂ is dissolved as a "carbonic acid," which may be combined with dissolved calcium to form solid calcium carbonate (limestone). Both precipitation and solution are influenced by the bacteria and plant roots in soils, where they improve gaseous circulation, or in coral reefs, where calcium carbonate is deposited as a solid on the sea floor. Calcium carbonate can also be washed from continents to the sea where it is used by living organisms to manufacture carbonaceous tests and shells. Once dead, the living organisms' shells fall to the bottom of the oceans where they generate deposits of chalk and limestone. Part of the organisms with carboneous shells are the coccolithophores (algae), which also happen to participate in the formation of clouds. When they die, they release a sulfurous gas (DMS), (CH₃)₂S, which act as particles on which water vapor condenses to make clouds.

Lovelock sees this as one of the complex processes that maintain conditions suitable for life. The volcanoes produce CO₂ in the atmosphere, CO₂ participates in rock weathering as carbonic acid, itself accelerated by temperature and soil life, the dissolved CO₂ is then used by the algae and released on the ocean floor. CO₂ excess can be compensated by an increase of coccolithophoride life, increasing the amount of CO₂ locked in the ocean floor. Coccolithophorides increase the cloud cover, hence control the surface temperature, help cool the whole planet and favor precipitations which are necessary for terrestrial plants. For Lovelock, coccolithophorides are one stage in a regulatory feedback loop. Lately the atmospheric CO₂ concentration has increased and there is some evidence that concentrations of ocean algal blooms are also increasing.

Critical analysis

Basis

This theory is based on the simple idea that the biomass self-regulates the conditions on the planet to make its physical environment (in particular temperature and chemistry of the atmosphere) on the planet more hospitable to the species which constitute its "life". The Gaia Hypothesis proper defined this "hospitality" as a full homeostasis. A simple model that is often used to illustrate the original Gaia Hypothesis is the so-called Daisyworld simulation.

Whether this sort of system is present on Earth is still open to debate. Some relatively simple homeostatic mechanisms are generally accepted. For example, when atmospheric carbon dioxide levels rise, plants are able to grow better and thus remove more carbon dioxide from the atmosphere, but the extent to which these mechanisms stabilize and modify the Earth's overall climate are not yet known. Less clear is the reason why such traits evolve in a system in order to produce such effects.

Criticism

After initially being largely ignored by scientists, (from 1969 till 1977), thereafter for a period, the initial Gaia hypothesis was ridiculed by some scientists. On the basis of its name alone, the Gaia hypothesis was derided as some kind of neo-Pagan New Age religion. Many scientists in particular also criticised the approach taken in his popular book "Gaia, a New look at Life on Earth" for being teleological; a belief that all things have a predetermined purpose. Lovelock seems to have accepted this criticism of some of his statements, and has worked hard to remove the taint of teleological purpose from his theories, stating "Nowhere in our writings do we express the idea that planetary self-regulation is purposeful, or involves foresight or planning by the biota." – (Lovelock, J. E. 1990).

In 1981, W. Ford Doolittle, in the CoEvolution Quarterly article "Is Nature Motherly" argued that there was nothing in the genome of individual organisms which could provide the feedback mechanisms Gaia theory proposed, and that therefore the Gaia hypothesis was an unscientific theory of a maternal type without any explanatory mechanism. In 1982 Richard Dawkins in his book The Blind Watchmaker argued that organisms could not act in concert as this would require foresight and planning from them. Like Doolittle he rejected the possibility that feedback loops could stabilize the system. Dawkins claimed "there was no way for evolution by natural selection to lead to altruism on a Global scale".

At the American Geophysical Union's first Chapman Conference on Gaia, held at San Diego in 1981, James Kirchner criticised the Gaia hypothesis for its imprecision. He claimed that Lovelock and Margulis had not presented one Gaia hypothesis, but four -

• CoEvolutionary Gaia - that life and the environment had evolved in a coupled way. Kirchner claimed that this was already accepted scientifically and was not new.
• Homeostatic Gaia - that life maintained the stability of the natural environment, and that this stability enabled life to continue to exist.
• Geophysical Gaia - that the Gaia theory generated interest in geophysical cycles and therefore led to interesting new research in terrestrial geophysical dynamics.
• Optimising Gaia - that Gaia shaped the planet in a way that made it an optimal environment for life as a whole. Kirchner claimed that this was not testable and therefore was not scientific.

Of Homeostatic Gaia, Kirchner recognised two alternatives. "Weak Gaia" asserted that life tends to make the environment stable for the flourishing of all life. "Strong Gaia" according to Kirchner, asserted that life tends to make the envronment stable, in order to enable the flourishing of all life. Strong Gaia, Kirchner claimed, was untestable and therefore not scientific.

Lawrence Joseph in his book "Gaia: the birth of an idea" demonstrated that Kirchner's attack was principally against Lovelock's integrity as a scientist, and was graciously ignored at the gathering by Lovelock himself. Nevertheless he and other Gaia inspired scientists have worked hard to disprove the claim that the theory is not scientific because it is impossible to test it by controlled experiment. Against the charge that Gaia was Teleological Lovelock and Andrew Watson offered the Daisyworld model as mathematical evidence to refute most of these criticisms.

A final criticism leveled against the idea that Gaia is a "living" organism is the fact that the planet has not and is unable to reproduce. Certainly one of the hallmarks of living organisms is their ability to replicate and pass on their genetic information to succeeding generations. But the same criticism could be levelled at a sterile mule, or of a post-menopausal female. We have no difficulty recognising the fact that they are alive even though they cannot reproduce. Other Gaians have proposed that Gaia is still too young to reproduce and this is not to say that it is conceptually impossible, as humankind may be the means by which Gaia will reproduce. Humanity's exploration of space, its interest in colonizing other planets, and the large body of sci-fi literature that describes terraforming, lend strong evidence to the idea that Gaia is planning to reproduce. The astronomer Carl Sagan also remarked that from a cosmic viewpoint, the space probes since 1959 have the character of a planet preparing to go to seed.

DaisyWorld simulations

Lovelock responded to criticisms with the mathematical Daisyworld model (1983), first to prove the existence of feedback mechanisms, second to demonstrate it was possible that control of the global biomass could occur without consciousness being involved.

More recently, studies of artificial life using various guilds, of photosynthesisers, herbivores, carnivores, scavengers and decomposers, enables nutrient recycling within a regulatory framework derived by natural selection amongst species, where one being's harmful waste, becomes low energy food for members of another guild. This research on the Redfield Ratio of Nitrogen to Phosphorus, shows that local biotic processes can regulate global systems (See Keith Downing & Peter Zvirinsky, The Stimulated Evolution of Biochemical Guilds: Reconciling Gaia Theory with Natural Selection).

The First Gaia Conference

In 1988, to draw attention to the Gaia hypothesis, the climatologist Stephen Schneider organised a conference of the American Geophysical Union solely to discuss Gaia. The accusations of teleologism were largely dropped after that meeting.

Lovelock was careful to present a version of the Gaia Hypothesis which had no claim that Gaia intentionally or consciously maintained the complex balance in her environment that life needed to survive. It would appear that the claim that Gaia acts "intentionally" was a metaphoric statement in his popular initial book and was not meant to be taken literally. This new statement of the Gaia hypothesis was more acceptable to the scientific community. He supported this hypothesis with the example of Daisyworld. Using computer simulations of a hypothetical Daisyworld (with no atmosphere, taking into account only the different albedos of a black and white daisy type) and a mathematical approach, Lovelock and Andrew Watson proved that the controlled stability of the climate was an automatic consequence of the feedback mechanisms that would foster one kind of daisy over another. Thus the Gaian stabilities were not being teleological. The Gaia hypothesis had always asserted that Gaia was homeostatic, i.e. that the biota influence the abiotic world in a way that involves homeostatic feedback. Later tests, using grey daisies in addition to black and white ones, from which mutations could lead to evolution of different colours, and the introduction of daisy eating rabbits and rabbit eating foxes only seemed to increase the stability of the Daisyword simulations. Kirchner responded that these results were predicatble because of the intention of the programmers - Lovelock and Watson, who selected examples which would produce the responses they desired. More recently other research since the first conference, modelling the real biochemical cycles of Earth, and using various "guilds" of life (eg. photosynthesisers, decomposers, herbivores and primary and secondary carnivores has also been shown to produce Daisyworld-like regulation and stability, which helps to explain planetary biological diversity.

Strong and weak Gaia hypotheses

Despite the evidence that Gaia Theory has developed considerably many critics have still not progressed beyond Kirchner's original attacks on Gaia Theory. They persist in stating that several types of strong Gaia hypotheses may be defined. A version called "Optimizing Gaia" asserts that biota manipulate their physical environment for the purpose of creating biologically favorable, or even optimal, conditions for themselves. "The Earth's atmosphere is more than merely anomalous; it appears to be a contrivance specifically constituted for a set of purposes" (Lovelock and Margulis, 1974). Further, "...it is unlikely that chance alone accounts for the fact that temperature, pH and the presence of compounds of nutrient elements have been, for immense periods, just those optimal for surface life. Rather, ... energy is expended by the biota to actively maintain these optima."

Another strong hypothesis is the one called "Omega Gaia". Teilhard de Chardin claimed that the Earth is evolving through stages of cosmogenesis, affecting the geosphere, biogenesis of the biosphere, and noogenesis of the noosphere, culminating in the Omega Point.

Weak Gaian hypotheses suggest that Gaia is co-evolutive. Co-evolution in this context has been thus defined: "Biota influence their abiotic environment, and that environment in turn influences the biota by Darwinian process." Lovelock (1995) gave evidence of this in his second book, showing the evolution from the world of the early thermo-acido-phyllic and methanogenic bacteria towards the oxygen enriched atmosphere today that supports more complex life.

The weakest form of the theory has been called "influential Gaia". It states that biota barely influence certain aspects of the abiotic world, e.g. temperature and atmosphere. Few would disagree.

The weak versions are more acceptable from an orthodox science perspective, as they assume non-homeostasis. They state the evolution of life and its environment may affect each other. An example is how the activity of photosynthetic bacteria during Precambrian times have completely modified the Earth atmosphere to turn it aerobic, and as such supporting evolution of life (in particular eukaryotic life) . However, these theories do not claim the atmosphere modification has been done in coordination and through homeostasis. Also such critical theories have yet to explain how conditions on Earth have not been changed by the kinds of run-away positive feedbacks that have effected Mars and Venus.

Unfortunately, some critics argue, many supporters of the various Gaia theories do not state exactly where they sit on this spectrum; this makes discussion and criticism difficult. More recently, after other conferences, the whole spectrum of "weak" and "strong Gaia" itself has been called into question. The "Gaia Hypothesis" has developed into "Gaia Theory" and even "Gaia Science" where a variety of different interpretations and modifications are simultaneously developing. Much effort on behalf of those analyzing the theory currently is an attempt to clarify what these different hypotheses are, and devising and presenting variours proposals that "test" the various outcomes.

Controversial concepts

Lovelock, especially in his older texts, has often indulged in language that has later caused fiery debates. What does he mean in the first paragraph of his first Gaia book (1979), when he writes that "the quest for Gaia is an attempt to find the largest living creature on Earth"? How "alive" is that, and in what respect? Is Gaia really an organism? In what sense? And in what sense does the cybernetic system called Gaia seek "an optimal physical and chemical environment for life on this planet"? Optimal for whom?

Lynn Margulis, the coauthor of Gaia hypotheses, is more careful to avoid controversial figures of speech than is Lovelock. In 1979 she wrote, in particular, that only homeorhetic and not homeostatic balances are involved: that is, the composition of Earth's atmosphere, hydrosphere, and lithosphere are regulated around "set points" as in homeostasis, but those set points change with time. Also she wrote that there is no special tendency of biospheres to preserve their current inhabitants, and certainly not to make them comfortable. Accordingly, the Earth is not a living organism which can live or die all at once, but rather a kind of community of trust which can exist at many discrete levels of integration. But this is true of all multicellular organisms, not all cells in the body die instantaneously.

In her 1998 book, The Symbiotic Planet, Margulis dedicated the last of the book's eight chapters to Gaia. She resented the widespread personification of Gaia and stressed that Gaia is "not an organism", but "an emergent property of interaction among organisms". She defined Gaia "the series of interacting ecosystems that compose a single huge ecosystem at the Earth's surface. Period." Yet still she argues, "the surface of the planet behaves as a physiological system in certain limited ways". Margulis seems to agree with Lovelock in that, in what comes to these physiological processes, the earth's surface is "best regarded as alive". The book's most memorable "slogan" was actually quipped by a student of Margulis': "Gaia is just symbiosis as seen from space". This neatly connects Gaia theory to Margulis' own theory of endosymbiosis.

Both Lovelock's and Margulis's understanding of Gaia are now largely considered valid scientific hypotheses, though controversies continue.

Recent developments

The Second Gaia Conference

By the time of the 2nd Chapman Conference on the Gaia Hypothesis, held at Valencia, in Spain on the 23 June 2000, the situation had developed significantly in accordance with the developing science of Bio-geophysiology. Rather than a discussion of the Gaian teleological views, or "types" of Gaia Theory, the focus was upon the specific mechanisms by which basic short term homeostasis was maintained within a framework of significant evolutionary long term structural change.

The major questions were:--

A. "How has the global biogeochemical/climate system called Gaia changed in time? What is its history? Can Gaia maintain stability of the system at one time scale but still undergo vectorial change at longer time scales? How can the geologic record be used to examine these questions?

B. What is the structure of Gaia? Are the feedbacks sufficiently strong to influence the evolution of climate? Are there parts of the system determined pragmatically by whatever disciplinary study is being undertaken at any given time or are there a set of parts that should be taken as most true for understanding Gaia as containing evolving organisms over time? What are the feedbacks among these different parts of the Gaian system, and what does the near closure of matter mean for the structure of Gaia as a global ecosystem and for the productivity of life?

C. How do models of Gaian processes and phenomena relate to reality and how do they help address and understand Gaia? How do results from Daisyworld transfer to the real world? What are the main candidates for "daisies"? Does it matter for Gaia theory whether we find daisies or not? How should we be searching for daisies, and should we intensify the search? How can Gaian mechanisms be investigated using process models or global models of the climate system which include the biota and allow for chemical cycling?"

Tyler Volk (1997) has suggested that once life evolves, a Gaian system is almost inevitably produced as a result of an evolution towards far-from-equilibrium homeostatic states that maximise entropy production (MEP). Kleidon (2004) agrees with Volk's hypothesis, stating: "...homeostatic behavior can emerge from a state of MEP associated with the planetary albedo"; "...the resulting behavior of a biotic Earth at a state of MEP may well lead to near-homeostatic behavior of the Earth system on long time scales, as stated by the Gaia hypothesis." Staley (2002) has similarly proposed "...an alternative form of Gaia theory based on more traditional Darwinian principles... In [this] new approach, environmental regulation is a consequence of population dynamics, not Darwinian selection. The role of selection is to favor organisms that are best adapted to prevailing environmental conditions. However, the environment is not a static backdrop for evolution, but is heavily influenced by the presence of living organisms. The resulting co-evolving dynamical process eventually leads to the convergence of equilibrium and optimal conditions."

Global warming inevitable

In Lovelock's latest book, The Revenge of Gaia, he argues that because of global warming the world population should brace itself for the inevitable: most of the planet will become uninhabitable by the end of this century (Anon., 2006).

Gaia hypothesis in ecology

After much initial criticism, a modified Gaia hypothesis is now considered within ecological science basically consistent with the planet earth being the ultimate object of ecological study. Ecologists generally consider the biosphere as an ecosystem and the Gaia hypothesis, though a simplification of that original proposed, to be consistent with a modern vision of global ecology, relaying the concepts of biosphere and biodiversity. The Gaia hypothesis has been called geophysiology or Earth system science, which takes into account the interactions between biota, the oceans, the geosphere, and the atmosphere.

Other Notes

At least three works of fiction use the Gaia hypothesis as a central part of the plot. The film Final Fantasy: The Spirits Within, arguably contains a fictional parallel to Sir James Lovelock in the character of Dr. Sid, who is met with skepticism from the scientific and social community when he promotes the idea of a "living Earth". In the film, Dr. Sid attempts to create a "waveform" from the positive energy signature of the Earth's spirit, in order to combat the films antagonists, the negative energy "Phantoms", through use of phase inversion canceling.

In Lovelock (1994), a novel by Orson Scott Card & Kathryn H. Kidd), Gaiaology is a fully-fledged interdisciplinary science which will soon be put to use by the Earth's first interstellar colony ship. Assuming the target planet will need terraforming, the job of the ship's Gaiaologist will be to integrate the terrestrial species needed for the colonists' survival with the planet's existing ecology. The Gaiaologist's "Witness," a form of assistance animal whose job it is to record every waking moment in the life of such a prominent member of society, is the central character of the book, an enhanced Capuchin monkey named after Sir James Lovelock.

The Gaia hypothesis was also used as a central theme in the novel Portent, by James Herbert, in which Lovelock is mentioned by name.

Maxis has specifically named the Gaia hypothesis and Lovelock as inspirations for their 1990 game, SimEarth.

Fritjof Capra in his fourth book The web of life too has used Gaia theory to explain the complications and interconnections in the marvellous web of life.

An oratorio by American composer Nathan Currier called Gaian Variations was premiered on Earth Day 2004 at Lincoln Center by the Brooklyn Philharmonic, using texts of James Lovelock, Loren Eiseley and Lewis Thomas.

See also

• Arcology
• Climate engineering
• Earth Charter
• Environmentalism
• Elisabet Sahtouris
• Gaia (Foundation universe)
• Geophysiology
• Industrial ecology
• James Kirchner
• Noosphere
• Odic force
• Places to intervene in a system
• SimEarth
• Solaris
• Technogaianism
• Urban ecology
• Vladimir Vernadsky intellectual precursor to Gaia theory

References

• Anonymous (2006). The Independent. on-line article, 16 January 2006.
• Kleidon, Axel (2004). Beyond Gaia: Thermodynamics of Life and Earth system functioning. Climate Change, 66(3): 271-319.
• Lovelock, James (2000). Gaia: A New Look at Life on Earth ISBN 0192862189
• Lovelock, James (1995). The Ages of Gaia: A Biography of Our Living Earth ISBN 0393312399
• Lovelock, James (2001). Homage to Gaia: The Life of an Independent Scientist ISBN 0198604297
• Lovelock, James (2006). The Revenge of Gaia: Why the Earth Is Fighting Back - and How We Can Still Save Humanity. Allen Lane, Santa Barbara (California). ISBN 0713999144..
• Margulis, Lynn (1998). Symbiotic Planet: A New Look at Evolution. Weidenfeld & Nicolson, London. ISBN 029781740X.
• Staley, M. (2004). Darwinian selection leads to Gaia. J. Theoretical Biol., 218(1): Staley abstract
• Stephen H. Schneider, et al.,(Eds) (2004), Scientists Debate Gaia: The Next Century ISBN 0262194988
• Thomas, Lewis (1974) Lives of a Cell
• Turney, Jim (2003). Lovelock & Gaia. Signs of Live. Icon Books UK, Cambridge. ISBN 1840464585.
• Volk, Tyler (1997). Gaia's Body: Towards a Physiology of the Earth ISBN 0387982701

External links

• Earthdance by Elisabet Sahtouris
• The Gaia Hypothesis proposed by Dr James Lovelock & Dr Lynn Margulis
• Gaia and Thermodynamics
• GaianVariations
• The Gaia Hypothesis
• Gaia Theory: Science of the Living Earth
• Keep a positive attitude Comment by Peter van Vliet of the iNSnet Foundation on Dr James Lovelock's article in The Independent of January 16 2006