The Gaia hypothesis, today also commonly referred to as Gaia theory, proposes that organisms interact with their inorganic surroundings on Earth to form a self-regulating, complex system that contributes to maintaining the conditions for life on the planet. The hypothesis was begun in 1965 by chemist James Lovelock and co-developed with microbiologist Lynn Margulis in the 1970s. The first paper co-authored by Lovelock and Margulis, which was the first significant published presentation of the hypothesis (1974), stated, “This paper examines the hypothesis that the total ensemble of living organisms which constitute the biosphere can act as a single entity to regulate the chemical composition, surface pH and possibly also climate.” The term "Gaia" itself was described as follows in this early paper, “Hence forward the word Gaia will be used to describe the biosphere and all of those parts of the Earth with which it actively interacts,” but it was not until later in the following decade, after considerable criticism from neo-Darwinian biologists, that it was made clear that the self-regulation arising from this totality was an emergent property of the whole system (today commonly referred to as the 'Earth System') and not stemming from organisms alone.
In 2001, more than 1,000 scientists from over 100 countries under the auspices of the United Nations, representing four different global research bodies, together signed the Amsterdam Declaration on Global Change, the first point of which begins, “The Earth System behaves as a single, self-regulating system,” suggesting that the concept of planetary self-regulation, when defined as regulation of the whole Earth System, has now become the conventional knowledge of science, leading some to feel that the Gaia hypothesis should be elevated to the status of a full-blown scientific theory. In 2006, the Geological Society of London awarded Lovelock the Wollaston Medal largely for his work on the Gaia theory.
The hypothesis was initially criticized for being teleological and contradicting principles of natural selection, and some scientists still consider it to be at odds with, or weakly supported by, available evidence, but later refinements resulted in ideas framed by the Gaia hypothesis being applied in a variety of other fields, and considerable overlap now exists between Gaia theory and such fields as Earth system science, biogeochemistry, systems ecology, and the emerging subject of geophysiology.
- 1 Introduction
- 2 Details
- 3 History
- 4 Criticism
- 5 See also
- 6 References
- 7 Notes
- 8 External links
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The scientifically accepted form of the hypothesis has been called "influential Gaia". It states the biota influence certain aspects of the abiotic world, e.g. temperature and atmosphere. 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's atmosphere to turn it aerobic, and as such supporting evolution of life (in particular eukaryotic life).
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 may have counterbalancing effects on environmental change. Opponents of this view sometimes reference examples of events that 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.
A less accepted version of the hypothesis, known as "strong Gaia," claims that changes in the biosphere are brought about through the coordination of living organisms and maintain those conditions through homeostasis. In some versions of Gaia philosophy, all lifeforms are considered 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.
The Gaia theory posits that the Earth is a self-regulating complex system involving the biosphere, the atmosphere, the hydrospheres and the pedosphere, tightly coupled as an evolving system. The theory sustains that this system as a whole, called Gaia, seeks a physical and chemical environment optimal for contemporary life.
Gaia evolves through a cybernetic feedback system operated unconsciously by the biota, leading to broad stabilization of the conditions of habitability in a full homeostasis. Many processes in the Earth's surface essential for the conditions of life depend on the interaction of living forms, especially microorganisms, with inorganic elements. These processes establish a global control system that regulates Earth's surface temperature, atmosphere composition and ocean salinity, powered by the global thermodynamic disequilibrium state of the Earth system.
The existence of a planetary homeostasis influenced by living forms had been observed previously in the field of biogeochemistry, and it is being investigated also in other fields like Earth system science. The originality of the Gaia theory relies on the assessment that such homeostatic balance is actively pursued with the goal of keeping the optimal conditions for life, even when terrestrial or external events menace them.
Regulation of the salinity in the oceans
Ocean salinity has been constant at about 3.4% for a very long time. Salinity stability in oceanic environments is important as most cells require a rather constant salinity and do not generally tolerate values above 5%. The constant ocean salinity was a long-standing mystery, because no process counterbalancing the salt influx from rivers was known. 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 mid-ocean ridges. However, the composition of seawater is far from equilibrium, and it is difficult to explain this fact without the influence of organic processes. One suggested explanation lies in the formation of salt plains throughout Earth's history. It is hypothesized that these are created by bacterial colonies that fix ions and heavy metals during their life processes.
Regulation of oxygen in the atmosphere
The atmospheric composition remains fairly constant, providing the conditions that contemporary life has adapted to. All the atmospheric gases other than noble gases present in the atmosphere are either made by organisms or processed by them. The Gaia theory states that the Earth's atmospheric composition is kept at a dynamically steady state by the presence of life.
The stability of the atmosphere in Earth is not a consequence of chemical equilibrium as it is in planets without life. Oxygen is the second most reactive electro-negative element after fluorine, and should combine with gases and minerals of the Earth's atmosphere and crust. Traces of methane (at an amount of 100,000 tonnes produced per year) should not exist, as methane is combustible in an oxygen atmosphere.
Dry air in the atmosphere of Earth contains roughly (by volume) 78.09% nitrogen, 20.95% oxygen, 0.93% argon, 0.039% carbon dioxide, and small amounts of other gases including methane. Lovelock originally speculated that concentrations of oxygen above about 25% would increase the frequency of wildfires and conflagration of forests. Recent work on the findings of fire-caused charcoal in Carboniferous and Cretaceous coal measures, in geologic periods when O2 did exceed 25%, has supported Lovelock's contention.
Regulation of the global surface temperature
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 within the levels of habitability, reaching quite regular low and high margins. Lovelock has also hypothesised that methanogens produced elevated levels of methane in the early atmosphere, giving a view similar to that found in petrochemical smog, similar in some respects to the atmosphere on Titan. This, Lovelock suggests, tended to screen out ultraviolet until the formation of the ozone screen, maintaining a degree of homeostasis. However, the Snowball Earth research has suggested that "oxygen shocks" and reduced methane levels led, during the Huronian, Sturtian and Marinoan/Varanger Ice Ages, to a world that very nearly became a solid "snowball". These epochs are evidence against the ability of the biosphere to fully self-regulate. Processing of the greenhouse gas CO2, explained below, plays a critical role in the maintenance of the Earth temperature within the limits of habitability.
The CLAW hypothesis, inspired by Gaia theory, proposes a feedback loop that operates between ocean ecosystems and the Earth's climate. The hypothesis specifically proposes that particular phytoplankton that produce dimethyl sulfide are responsive to variations in climate forcing, and that these responses lead to a negative feedback loop that acts to stabilise the temperature of the Earth's atmosphere. CLAW has stimulated a great deal of research and generated hundreds of peer reviewed papers, although without conclusive results. A review article published in 2011 in Nature (Quinn & Bates, 2011), claimed that, while CLAW had spawned 25 years of valuable research, it was “time to retire the CLAW hypothesis.” But Quinn & Bates neglected to mention the significant Thomas et al 2010 study published eighteen months earlier, which, its authors noted, “represents the first such model analysis to investigate the impact on cloud microphysics and climate using a GCM with coupled aerosol-chemistry.”
Thomas et al noted that their study, “realistically simulates the seasonality in the number of activated particles and CDNC,” in their target area of the Southern oceans, and their modeling suggests a total global radiative forcing from DMS of about -2W/m2. What is clearly important in Quinn & Bates is that the initial CLAW hypothesis has been made more complex by other competing CCN sources – sea salt and other organics – as well as by issues of spatial decoupling. As they note, “The spatial decoupling between DMS production and entrainment of DMS-derived particles into the MBL prevents a local marine biota–climate feedback loop.”
This spatial decoupling might, on the other hand, also be one of the reasons for the mixed results of many field studies, leading to an underestimation of DMS climate effects, aside from complicating the loop. As Thomas et al note, “A perturbed DMS patch in the southern oceans induces high CCN concentrations several thousand kilometers downwind of the patch due to the time scale (several days) of conversion from DMS into CCN (Woodhouse et al., 2008),” and that this possibly “complicates the interpretation of the field measurements (Korhonen et al., 2008).”
The question of whether there could, as a result of such spatial decoupling, be no feedback loop closure, is more complicated. One might say that the evolution of CLAW mirrors that of Gaia theory as a whole: Gaia progressed from an early hypothesis suggesting biotic regulation to a mature theory describing an emergent property of the entire Earth System, and CLAW effects, similarly, are likely to be complex and involve both biotic and abiotic factors together, and any feedbacks probably could not be driven by advantages registered through the simple ‘cost-benefit’ analyses of Neo-Darwinian biologists.
Currently the increase in human population and the environmental impact of their activities, such as the multiplication of greenhouse gases may cause negative feedbacks in the environment to become positive feedback. Lovelock has stated that this could bring an extremely accelerated global warming, but he has since stated the effects will likely occur more slowly.
James Lovelock and Andrew Watson developed the mathematical model Daisyworld, in which temperature regulation arises from a simple ecosystem consisting of two species whose activity varies in response to the planet's environment. The model demonstrates that beneficial feedback mechanisms can emerge in this "toy world" containing only self-interested organisms rather than through classic group selection mechanisms.
Daisyworld examines the energy budget of a planet populated by two different types of plants, black daisies and white daisies. The colour of the daisies influences the albedo of the planet such that black daisies absorb light and warm the planet, while white daisies reflect light and cool the planet. As the model runs the output of the "sun" increases, meaning that the surface temperature of an uninhabited "gray" planet will steadily rise. In contrast, on Daisyworld competition between the daisies (based on temperature-effects on growth rates) leads to a shifting balance of daisy populations that tends to favour a planetary temperature close to the optimum for daisy growth.
It has been suggested that the results were predictable because Lovelock and Watson selected examples that produced the responses they desired.
Processing of CO2
Gaia scientists see the participation of living organisms in the carbon cycle as one of the complex processes that maintain conditions suitable for life. 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. Carbon precipitation, solution and fixation 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 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.
One of these organisms is Emiliania huxleyi, an abundant coccolithophore algae which also has a role in the formation of clouds. CO2 excess is 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 necessary for terrestrial plants. Lately the atmospheric CO2 concentration has increased and there is some evidence that concentrations of ocean algal blooms are also increasing.
Lichen and other organisms accelerate the weathering of rocks in the surface, while the decomposition of rocks also happens faster in the soil, thanks to the activity of roots, fungi, bacteria and subterranean animals. The flow of carbon dioxide from the atmosphere to the soil is therefore regulated with the help of living beings. When CO2 levels rise in the atmosphere the temperature increases and plants grow. This growth brings higher consumption of CO2 by the plants, who process it into the soil, removing it from the atmosphere.
The idea of the Earth as an integrated whole, a living being, has a long tradition. The mythical Gaia was the primal Greek goddess personifying the Earth, the Greek version of "Mother Nature", or the Earth Mother. James Lovelock gave this name to his hypothesis after a suggestion from the novelist William Golding, who was living in the same village as Lovelock at the time (Bowerchalke, Wiltshire, UK). Golding's advice was based on Gea, an alternative spelling for the name of the Greek goddess, which is used as prefix in geology, geophysics and geochemistry. Golding later made reference to Gaia in his Nobel prize acceptance speech.
In the eighteenth century, as geology consolidated as a modern science, James Hutton maintained that geological and biological processes are interlinked. Later, the naturalist and explorer Alexander von Humboldt recognized the coevolution of living organisms, climate, and Earth's crust. In the twentieth century, Vladimir Vernadsky formulated a theory of Earth's development that is now one of the foundations of ecology. The Ukrainian geochemist was one of the first scientists to recognize that the oxygen, nitrogen, and carbon dioxide in the Earth's atmosphere result from biological processes. During the 1920s he published works arguing that living organisms could reshape the planet as surely as any physical force. Vernadsky was a pioneer of the scientific bases for the environmental sciences. His visionary pronouncements were not widely accepted in the West, and some decades after the Gaia hypothesis received the same type of initial resistance from the scientific community.
Also in the turn to the 20th century Aldo Leopold, pioneer in the development of modern environmental ethics and in the movement for wilderness conservation, suggested a living Earth in his biocentric or holistic ethics regarding land.
It is at least not impossible to regard the earth's parts—soil, mountains, rivers, atmosphere etc,—as organs or parts of organs of a coordinated whole, each part with its definite function. And if we could see this whole, as a whole, through a great period of time, we might perceive not only organs with coordinated functions, but possibly also that process of consumption as replacement which in biology we call metabolism, or growth. In such case we would have all the visible attributes of a living thing, which we do not realize to be such because it is too big, and its life processes too slow.
— Stephan Harding , Animate Earth.
Another influence for the Gaia theory and the environmental movement in general came as a side effect of the Space Race between the Soviet Union and the United States of America. During the 1960s, the first humans in space could see how the Earth looked as a whole. The photograph Earthrise taken by astronaut William Anders in 1968 during the Apollo 8 mission became an early symbol for the global ecology movement.
Formulation of the hypothesis
James Lovelock started defining the idea of a self-regulating Earth controlled by the community of living organisms in September 1965, while working at the Jet Propulsion Laboratory in California on methods of detecting life on Mars. The first paper to mention it was Planetary Atmospheres: Compositional and other Changes Associated with the Presence of Life, co-authored with C.E. Giffin. A main concept was that life could be detected in a planetary scale by the chemical composition of the atmosphere. According to the data gathered by the Pic du Midi observatory, planets like Mars or Venus had atmospheres in chemical equilibrium. This difference with the Earth atmosphere was considered to be a proof that there was no life in these planets.
Lovelock formulated the Gaia Hypothesis in journal articles in 1972 and 1974, followed by a popularizing 1979 book Gaia: A new look at life on Earth. An article in the New Scientist of February 6, 1975, and a popular book length version of the hypothesis, published in 1979 as The Quest for Gaia, began to attract scientific and critical attention.
Lovelock called it first the Earth feedback hypothesis, and it was a way to explain the fact that combinations of chemicals including oxygen and methane persist in stable concentrations in the atmosphere of the Earth. Lovelock suggested detecting such combinations in other planets' atmospheres as a relatively reliable and cheap way to detect life.
Later, other relationships such as sea creatures producing sulfur and iodine in approximately the same quantities as required by land creatures emerged and helped bolster the theory.
In 1971 microbiologist Dr. Lynn Margulis joined Lovelock in the effort of fleshing out the initial hypothesis into scientifically proven concepts, contributing her knowledge about how microbes affect the atmosphere and the different layers in the surface of the planet. The American biologist had also awakened criticism from the scientific community with her theory on the origin of eukaryotic organelles and her contributions to the endosymbiotic theory, nowadays accepted. Margulis dedicated the last of eight chapters in her book, The Symbiotic Planet, to Gaia. However, she objected to the widespread personification of Gaia and stressed that Gaia is "not an organism", but "an emergent property of interaction among organisms". She defined Gaia as "the series of interacting ecosystems that compose a single huge ecosystem at the Earth's surface. Period". The book's most memorable "slogan" was actually quipped by a student of Margulis': "Gaia is just symbiosis as seen from space".
James Lovelock called his first proposal the Gaia hypothesis but has also used the term Gaia theory. Lovelock states that the initial formulation was based on observation, but still lacked a scientific explanation. The Gaia hypothesis has since been supported by a number of scientific experiments and provided a number of useful predictions. In fact, wider research proved the original hypothesis wrong, in the sense that it is not life alone but the whole Earth system that does the regulating.
First Gaia conference
In 1985, the first public symposium on the Gaia hypothesis, Is The Earth A Living Organism? was held at University of Massachusetts Amherst, August 1–6. The principal sponsor was the National Audubon Society. Speakers included James Lovelock, George Wald, Mary Catherine Bateson, Lewis Thomas, John Todd, Donald Michael, Christopher Bird, Thomas Berry, David Abram, Michael Cohen, and William Fields. Some 500 people attended.
Second Gaia conference
During the "philosophical foundations" session of the conference, David Abram spoke on the influence of metaphor in science, and of Gaia theory as offering a new and potentially game-changing metaphorics, while James Kirchner criticised the Gaia hypothesis for its imprecision. Kirchner 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 environment stable, to enable the flourishing of all life. Strong Gaia, Kirchner claimed, was untestable and therefore not scientific.
Lovelock and other Gaia-supporting scientists, however, did attempt to disprove the claim that the theory is not scientific because it is impossible to test it by controlled experiment. For example, against the charge that Gaia was teleological, Lovelock and Andrew Watson offered the Daisyworld model (and its modifications, above) as evidence against most of these criticisms. Lovelock said that the Daisyworld model "demonstrates that self-regulation of the global environment can emerge from competition amongst types of life altering their local environment in different ways".
Lovelock was careful to present a version of the Gaia hypothesis that 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. Most accusations of teleologism ceased, following this conference.
Third Gaia conference
By the time of the 2nd Chapman Conference on the Gaia Hypothesis, held at Valencia, Spain, on 23 June 2000, the situation had changed significantly in accord 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:
- "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?"
- "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?"
- "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 that include the biota and allow for chemical cycling?"
In 1997, Tyler Volk argued that a Gaian system is almost inevitably produced as a result of an evolution towards far-from-equilibrium homeostatic states that maximise entropy production, and Kleidon (2004) agreed 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".
Fourth Gaia conference
A fourth international conference on the Gaia Theory, sponsored by the Northern Virginia Regional Park Authority and others, was held in October 2006 at the Arlington, VA campus of George Mason University.
Martin Ogle, Chief Naturalist, for NVRPA, and long-time Gaia Theory proponent, organized the event. Lynn Margulis, Distinguished University Professor in the Department of Geosciences, University of Massachusetts-Amherst, and long-time advocate of the Gaia Theory, was a keynote speaker. Among many other speakers: Tyler Volk, Co-director of the Program in Earth and Environmental Science at New York University; Dr. Donald Aitken, Principal of Donald Aitken Associates; Dr. Thomas Lovejoy, President of the Heinz Center for Science, Economics and the Environment; Robert Correll, Senior Fellow, Atmospheric Policy Program, American Meteorological Society and noted environmental ethicist, J. Baird Callicott. James Lovelock, the theory's progenitor, prepared a video for the event.
This conference approached Gaia Theory as both science and metaphor as a means of understanding how we might begin addressing 21st century issues such as climate change and ongoing environmental destruction.
After initially being largely ignored by most scientists (from 1969 until 1977), thereafter for a period the initial hypothesis was strongly criticized by a number of scientists, particularly prominent Neo-Darwinian biologists such as Ford Doolittle and Richard Dawkins. Lovelock has felt that by naming his theory after a Greek goddess, championed by many non-scientists, the Gaia hypothesis was easily dismissed as neo-Pagan 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 things are by purpose aimed towards a goal. Responding to this critique in 1990, Lovelock stated, "Nowhere in our writings do we express the idea that planetary self-regulation is purposeful, or involves foresight or planning by the biota". Stephen Jay Gould referred to Gaia as a "a metaphor, not a mechanism."  Lovelock argues, however, that no single mechanism is responsible, that the connections between the various known mechanisms may never be known, that this is accepted in other fields of biology and ecology as a matter of course. But by far the most pervasive criticisms of Gaia have come from the idea that it is incompatible with biological evolution and natural selection.
Natural selection and evolution
Lovelock has suggested that global biological feedback mechanisms could evolve by natural selection, stating that organisms that improve their environment for their survival do better than those that damage their environment. However, in 1981, W. Ford Doolittle, in the CoEvolution Quarterly article "Is Nature Really Motherly" argued that nothing in the genome of individual organisms could provide the feedback mechanisms proposed by Lovelock, and therefore the Gaia hypothesis proposed no plausible mechanism and was unscientific. In Richard Dawkins' 1982 book, The Extended Phenotype, he stated that for organisms to act in concert would require foresight and planning, which is contrary to the current scientific understanding of evolution. Like Doolittle, he also rejected the possibility that feedback loops could stabilize the system. Dawkins also stressed that the planet is not the offspring of any parents and is unable to reproduce.
Lynn Margulis, the co-developer of Gaia theory, was for decades at odds with the Neo-Darwinain view of evolution, quite aside from disputes concerning Gaia, and argued in 1999, that "Darwin's grand vision was not wrong, only incomplete. In accentuating the direct competition between individuals for resources as the primary selection mechanism, Darwin (and especially his followers) created the impression that the environment was simply a static arena". She wrote that the composition of the Earth's atmosphere, hydrosphere, and lithosphere are regulated around "set points" as in homeostasis, but that those set points evolve with time and thus actually create homeorhesis, and her book Symbiotic Planet aimed to tie together Gaia theory with her own work on the under-recognized role of symbiosis in evolution.
Evolutionary biologist W. D. Hamilton likened the concept of Gaia to the Copernican revolution, adding that it would now take another Newton to explain how Gaian self-regulation takes place through Darwinian natural selection.[better source needed]
Recent disputation concerning Gaia theory has generally remained deeply rooted in concerns of a basic conflict between the laws of natural selection and Gaia theory, much like the first objections raised by Dawkins and Doolittle, even as biology itself has been shifting under the feet of such arguments: for example, a series of intense exchanges about Gaia theory in the journal Climatic Change in 2002 and 2003 involving Lovelock, Lenton, Volk, Kleidon, Schwartzman, Kirchner and others continued to rotate around issues of natural selection, whether by-products of metabolism could be selected for, etc.
In 2013, Toby Tyrrell published a book, On Gaia: A Critical Investigation of the Relationship between Life and Earth, intended to objectively evaluate evidence for and against Gaia from across various relevant disciplines. Tyrrell found that the two weaker forms of Gaia — Coeveolutionary Gaia and Influential Gaia — were credible, but that it is not useful to use the term "Gaia" in this sense, and in a New Scientist paper published together with his book, concluded negatively, “The Gaia hypothesis is not an accurate picture of how our world works.”Tyrrell’s putative objectivity appears thin by the conclusion of his book, however: he ends by suggesting that Gaia theory could be “dangerously deceiving” by leading to excessive optimism about the planetary environment. He does acknowledge that Lovelock, in fact, was significant in the origins of modern environmental understanding, but neglects to mention how Lovelock has struggled against expressing excessive pessimism, not optimism, in recent years (i.e., The Revenge of Gaia, The Vanishing Face of Gaia). Further, Tyrrell argues at length against Gaia by trying to make the case that the Earth is in fact since the Pleistocene too cold for life, and should be warmer (based on questions of metabolism, land area available for colonization by the biota, etc.). Tyrrell’s anti-Gaia treatise, through such inadvertent means, in fact highlights the degree to which Gaia theory is in sync with the now-standard Earth system science.
Tyrrell’s 'evaluation', after its introductory chapter, similarly goes straight to arguments of 'Gaia versus natural selection,' in Good Citizens or Selfish Genes (Chapter 2), steeped in the rhetoric of kin selection theory, the role of cooperation in animal evolution, etc. Only a few months before Tyrrell's book was published, however, the Proceedings of the National Academy of Sciences published a major Perspectives paper, Animals in a bacterial world: a new imperative for the life sciences, by McFall-Ngai et al, involving almost thirty co-authors, which refers to “a vast and exciting frontier for the field of biology” that would “call on life scientists to alter significantly their view of the fundamental nature of the biosphere.” The paper does not concern Gaia theory. It does, however, directly involve the roots of the Neo-Darwinian framework which for more than thirty years has been fundamental to the central critique of Gaia theory. As McFall-Ngai et al note,
“For much of her professional career, Lynn Margulis (1938–2011), a controversial visionary in biology, predicted that we would come to recognize the impact of the microbial world on the form and function of the entire biosphere, from its molecular structure to its ecosystems. The weight of evidence supporting this view has finally reached a tipping point. The examples come from animal–bacterial interactions, as described here, and also from relationships between and among viruses, Archaea, protists, plants, and fungi. These new data are demanding a reexamination of the very concepts of what constitutes a genome, a population, an environment, and an organism. Similarly, features once considered exceptional, such as symbiosis, are now recognized as likely the rule, and novel models for research are emerging across biology. As a consequence, the New Synthesis of the 1930s and beyond must be reconsidered in terms of three areas in which it has proven weakest: symbiosis, development, and microbiology (115). One of these areas, microbiology, presents particular challenges both to the species concept, as formulated by Ernst Mayr in 1942, and to the concept that vertical transmission of genetic information is the only motor of selectable evolutionary change."
In such a radically altered view of biological evolution, apparent conflicts between Gaian-style planetary self-regulation and biological evolution will doubtless appear transformed. Indeed, it might be appropriate to add a fourth category of weakness since the New Synthesis in need of reconsideration by biologists to those three listed by the authors - namely, planetary self-regulation, which is likely incomprehensible within its current framework. As W.D. Hamilton noted, we will now need a new Newton to figure out how Gaian self-regulation works, and it appears likely that work Margulis began in Symbiotic Planet will become a primary point of inspiration for such research.
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