gaia hypothesis definition

Gaia Hypothesis

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• Ricardo Amils 11  

According to the Gaia Hypothesis, the biosphere and the physical components of the Earth form a complex interacting system that maintains the climatic and biogeochemical Earth conditions in homeorhesis. It was originally proposed by James Lovelock who called it the Earth feedback hypothesis, and it is frequently described as a way of seeing the Earth as a single organism.

James Lovelock first formulated the Gaia Hypothesis in the 1960s, as a result of his work for NASA on developing methods of detecting life on Mars. At first, the theory was a way to explain the stable concentrations of chemicals such as oxygen and methane that persisted in Earth’s atmosphere. Lovelock suggested that detecting such unstable combinations in other planets’ atmospheres was a relatively reliable and cheap way to detect life.

Lovelock tried to explain the existence of a global control system over surface temperature, the salinity of the oceans, and the atmospheric composition suggesting...

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References and Further Reading

Lovelock JE (1965) A physical basis for life detection experiments. Nature 207(7):568–570. https://doi.org/10.1038/207568a0

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Lovelock JE (1972) Gaia as seen through the atmosphere. Atmos Environ 6(8):579–580. https://doi.org/10.1016/0004-6981(72)90076-5

Lovelock JE (1990) Hands up for the Gaia hypothesis. Nature 344:100–102. https://doi.org/10.1038/344100a0

Lovelock JE (1995) The ages of Gaia: a biography of our living earth. Norton, New York. ISBN 0-393-31239-9

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Lovelock JE (2000) Gaia: a new look at life on earth. Oxford University Press, Oxford. ISBN 0-19-286218-9

Lovelock JE, Margulis L (1974) Atmospheric homeostasis by and for the biosphere- the Gaia hypothesis. Tellus 26(1):2–10

Margulis L (1999) Symbiotic planet: a new look at evolution. Basic Book, Houston

Schwartzman D (2002) Life, temperature, and the earth: the self-organizing biosphere. Columbia University Press, New York. ISBN 0231102135

Volk T (2003) Gaia’s body: toward a physiology of earth. MIT Press, Cambridge. ISBN 0-262-72042-6

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Departamento de Biologia Molecular, Universidad Autónoma de Madrid, Madrid, Spain

Ricardo Amils

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Laboratoire d’Astrophysique de Bordeaux, University of Bordeaux, Pessac, France

Muriel Gargaud

Department of Astronomy, University of Massachusetts, Amherst, MA, USA

William M. Irvine

Centro Biología Molecular Severo Ochoa, Universidad Autónoma de Madrid Cantoblanco, Madrid, Spain

Analytical, Environmental, Geo-Chemistry, Vrije Universiteit Brussel (VUB), Brussels, Belgium

Philippe Claeys

Earth-Life Science Institute, Tokyo Institute of Technology, Tokyo, Japan

Henderson James Cleaves

Radio Astronomy, Paris Observatory, Paris, France

Maryvonne Gerin

LESIA, Observatoire de Paris-Site de Meudon, Meudon, France

Daniel Rouan

International Space Science Institute, Bern, Switzerland

Tilman Spohn

Faculté des Sciences et des Techniques de Nantes, Centre François Viète d’Histoire des Sciences et de Techniques EA 1161, Nantes, France

Stéphane Tirard

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Michel Viso

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Amils, R. (2023). Gaia Hypothesis. In: Gargaud, M., et al. Encyclopedia of Astrobiology. Springer, Berlin, Heidelberg. https://doi.org/10.1007/978-3-662-65093-6_613

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The Gaia hypothesis

The evolution of life and the atmosphere.

• The biosphere and Earth’s energy budget
• The cycling of biogenic atmospheric gases
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• Biosphere controls on minimum temperatures
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• The effect of vegetation patchiness on mesoscale climates
• Biosphere controls on surface friction and localized winds
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• Biogenic ice nuclei
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The notion that the biosphere exerts important controls on the atmosphere and other parts of the Earth system has increasingly gained acceptance among earth and ecosystem scientists. While this concept has its origins in the work of American oceanographer Alfred C. Redfield in the mid-1950s, it was English scientist and inventor James Lovelock that gave it its modern currency in the late 1970s. Lovelock initially proposed that the biospheric transformations of the atmosphere support the biosphere in an adaptive way through a sort of “genetic group selection .” This idea generated extensive criticism and spawned a steady stream of new research that has enriched the debate and advanced both ecology and environmental science . Lovelock called his idea the “ Gaia Hypothesis ” and defined Gaia as

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

The Greek word Gaia, or Gaea, meaning “Mother Earth,” is Lovelock’s name for Earth, which is envisioned as a “superorganism” engaged in planetary biogeophysiology. The goal of this superorganism is to produce a homeostatic , or balanced, Earth system. The scientific process of research and debate will eventually resolve the issue of the reality of the “Gaian homeostatic superorganism,” and Lovelock has since revised his hypothesis to exclude goal-driven genetic group selection . Nevertheless, it is now an operative norm in contemporary science that the biosphere and the atmosphere interact in such a way that an understanding of one requires an understanding of the other. Furthermore, the reality of two-way interactions between climate and life is well recognized.

Life on Earth began at least as early as 3.5 billion years ago during the middle of the Archean Eon (about 4 billion to 2.5 billion years ago). It was during this interval that life first began to exercise certain controls on the atmosphere. The atmosphere’s prebiological state is often characterized as being rich in water vapour and carbon dioxide. Though some nitrogen was also present, little if any oxygen was available. Chemical reactions with hydrogen sulfide , hydrogen , and reduced compounds of nitrogen and sulfur precluded any but the shortest lifetime for free oxygen in the atmosphere. As a result, life evolved in an atmosphere that was reducing (high hydrogen content) rather than oxidizing (high oxygen content). In addition to their chemically reducing character, the predominant gases of this prebiotic atmosphere, with the exception of nitrogen, were largely transparent to incoming sunlight but opaque to outgoing terrestrial infrared radiation . As a result, these gases are called, perhaps improperly, greenhouse gases ( see greenhouse effect ) because they are able to slow the release of outgoing radiation back into space .

In the Archean Eon, the Sun produced as much as 25 percent less light than it does today; however, Earth’s temperature was much like that of today. This is possible because the greenhouse gas -rich Archean atmosphere was effective in retarding the loss of terrestrial radiation to space. The resulting long residence time of energy within the Earth-atmosphere system resulted in a warmer atmosphere than would have been possible otherwise. The average temperature of Earth’s surface in the early Archean Eon was warmer than the modern global average. It was, according to some sources, probably similar to temperatures found in today’s tropics. Depending on the amount of nitrogen present during the Archean Eon, it has been suggested that the atmosphere may have held more than 1,000 times as much carbon dioxide than it does today.

Archean organisms included photosynthetic and chemosynthetic bacteria , methane -producing bacteria, and a more primitive group of organisms now called the “ Archaea ” (a group of prokaryotes more related to eukaryotes than to bacteria and found in extreme environments). Through their metabolic processes, organisms of the Archean Eon slowly changed the atmosphere. Hydrogen rose from trace amounts to about 1 part per million (ppm) of dry air . Methane concentrations increased from near zero to about 100 ppm. Oxygen increased from near zero to 1 ppm, whereas nitrogen concentrations rose to encompass 99 percent of all atmospheric molecules excluding water vapour. Carbon dioxide concentrations decreased to only 0.3 percent of the total; however, this was nearly 10 times the current concentration. The composition of the atmosphere, its radiation budget, its thermodynamics , and its fluid dynamics were transformed by life from the Archean Eon.

American geochemist Robert Garrels calculated that, in the absence of life and given the burial rate of carbon in rocks , oxygen would be unavailable to form water, and free hydrogen would be lost to space. Without the presence of life and compounded by this loss of hydrogen, there would be no oceans , and Earth would have become merely a dusty planet by the middle of the Archean Eon. By the end of the Archean Eon 2.5 billion years ago, both the pigment chlorophyll and photosynthetic organisms had evolved such that the production of oxygen increased rapidly. The atmosphere became transformed from a reducing atmosphere with carbon dioxide, limited oxygen, and anaerobic organisms (that is, life-forms that do not require oxygen for respiration) in control to one with an oxidizing atmosphere that was rich in oxygen , poor in carbon dioxide, and dominated by aerobic organisms (that is, life-forms requiring oxygen for respiration).

With the decline in carbon dioxide and a rise in oxygen, the greenhouse warming capacity of Earth’s atmosphere was sharply reduced; however, this happened over a period of time when the energy produced by the Sun increased systematically. These compensating changes resulted in a relatively constant planetary temperature over much of Earth’s history .

Scientists finally have an explanation for the ‘Gaia puzzle’

Associate Professor of Sustainability Science, University of Southampton

Director, Global Systems Institute, University of Exeter

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We will likely never know how life on Earth started. Perhaps in a shallow sunlit pool. Or in the crushing ocean depths miles beneath the surface near fissures in the Earth’s crust that spewed out hot mineral-rich soup. While there is good evidence for life at least 3.7 billion years ago , we don’t know precisely when it started.

But these passing aeons have produced something perhaps even more remarkable: life has persisted. Despite massive asteroid impacts, cataclysmic volcano activity and extreme climate change, life has managed to not just cling on to our rocky world but to thrive.

How did this happen? Research we recently published with colleagues in Trends in Ecology and Evolution offers an important part of the answer, providing a new explanation for the Gaia hypothesis.

Developed by scientist and inventor James Lovelock , and microbiologist Lynn Margulis , the Gaia hypothesis originally proposed that life, through its interactions with the Earth’s crust, oceans, and atmosphere, produced a stabilising effect on conditions on the surface of the planet – in particular the composition of the atmosphere and the climate. With such a self-regulating process in place, life has been able to survive under conditions which would have wiped it out on non-regulating planets.

Lovelock formulated the Gaia hypothesis while working for NASA in the 1960s. He recognised that life has not been a passive passenger on Earth. Rather it has profoundly remodelled the planet, creating new rocks such as limestone, affecting the atmosphere by producing oxygen, and driving the cycles of elements such as nitrogen, phosphorus and carbon. Human-produced climate change, which is largely a consequence of us burning fossil fuels and so releasing carbon dioxide, is just the latest way life affects the Earth system.

While it is now accepted that life is a powerful force on the planet, the Gaia hypothesis remains controversial. Despite evidence that surface temperatures have, bar a few notable exceptions, remained within the range required for widespread liquid water, many scientists attribute this simply to good luck. If the Earth had descended completely into an ice house or hot house (think Mars or Venus) then life would have become extinct and we would not be here to wonder about how it had persisted for so long. This is a form of anthropic selection argument that says there is nothing to explain.

Clearly, life on Earth has been lucky. In the first instance, the Earth is within the habitable zone – it orbits the sun at a distance that produces surface temperatures required for liquid water. There are alternative and perhaps more exotic forms of life in the universe, but life as we know it requires water. Life has also been lucky to avoid very large asteroid impacts. A lump of rock significantly larger than the one that lead to the demise of the dinosaurs some 66m years ago could have completely sterilised the Earth.

But what if life had been able to push down on one side of the scales of fortune? What if life in some sense made its own luck by reducing the impacts of planetary-scale disturbances? This leads to the central outstanding issue in the Gaia hypothesis: how is planetary self-regulation meant to work?

While natural selection is a powerful explanatory mechanism that can account for much of the change we observe in species over time, we have been lacking a theory that could explain how the living and non-living elements of a planet produce self-regulation. Consequently the Gaia hypothesis has typically been considered as interesting but speculative – and not grounded in any testable theory .

Selecting for stability

We think there is finally an explanation for the Gaia hypothesis. The mechanism is based on “ sequential selection ”, a concept first suggested by climate scientist Richard Betts in the early 2000s. In principle it’s very simple. As life emerges on a planet it begins to affect environmental conditions, and this can organise into stabilising states which act like a thermostat and tend to persist, or destabilising runaway states such as the snowball Earth events that nearly extinguished the beginnings of complex life more than 600m years ago.

If it stabilises then the scene is set for further biological evolution that will in time reconfigure the set of interactions between life and planet. A famous example is the origin of oxygen-producing photosynthesis around 3 billion years ago, in a world previously devoid of oxygen. If these newer interactions are stabilising, then the planetary-system continues to self-regulate. But new interactions can also produce disruptions and runaway feedbacks. In the case of photosynthesis it led to an abrupt rise in atmospheric oxygen levels in the “ Great Oxidation Event ” around 2.3 billion years ago. This was one of the rare periods in Earth’s history where the change was so pronounced it probably wiped out much of the incumbent biosphere, effectively rebooting the system.

The chances of life and environment spontaneously organising into self-regulating states may be much higher than you would expect. If fact, given sufficient biodiversity, it may be extremely likely . But there is a limit to this stability. Push the system too far and it may go beyond a tipping point and rapidly collapse to a new and potentially very different state.

This isn’t a purely theoretical exercise, as we think we may able to test the theory in a number of different ways. At the smallest scale that would involve experiments with diverse bacterial colonies. On a much larger scale it would involve searching for other biospheres around other stars which we could use to estimate the total number of biospheres in the universe – and so not only how likely it is for life to emerge, but also to persist.

The relevance of our findings to current concerns over climate change has not escaped us. Whatever humans do life will carry on in one way or another. But if we continue to emit greenhouse gasses and so change the atmosphere, then we risk producing dangerous and potentially runaway climate change. This could eventually stop human civilisation affecting the atmosphere, if only because there will not be any human civilisation left.

Gaian self-regulation may be very effective. But there is no evidence that it prefers one form of life over another. Countless species have emerged and then disappeared from the Earth over the past 3.7 billion years. We have no reason to think that Homo sapiens are any different in that respect.

This article was updated on July 10 to add the reference to Richard Betts.

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The Gaia hypothesis (/ˈɡaɪ.ə/), also known as the Gaia theory, Gaia paradigm, or the Gaia principle, proposes that living organisms interact with their inorganic surroundings on Earth to form a synergistic and self-regulating, complex system that helps to maintain and perpetuate the conditions for life on the planet. The hypothesis was formulated by the chemist James Lovelock and co-developed by the microbiologist Lynn Margulis in the 1970s. Lovelock named the idea after Gaia, the primordial goddess who personified the Earth in Greek mythology. The suggestion that the theory should be called "the Gaia hypothesis" came from Lovelock's neighbour, William Golding. In 2006, the Geological Society of London awarded Lovelock the Wollaston Medal in part for his work on the Gaia hypothesis. Topics related to the hypothesis include how the biosphere and the evolution of organisms affect the stability of global temperature, salinity of seawater, atmospheric oxygen levels, the maintenance of a hydrosphere of liquid water and other environmental variables that affect the habitability of Earth. The Gaia hypothesis was initially criticized for being teleological and against the principles of natural selection, but later refinements aligned the Gaia hypothesis with ideas from fields such as Earth system science, biogeochemistry and systems ecology. Even so, the Gaia hypothesis continues to attract criticism, and today many scientists consider it to be only weakly supported by, or at odds with, the available evidence.

1. Overview

Gaian hypotheses suggest that organisms co-evolve with their environment: that is, they "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, Ages of Gaia , showing the evolution from the world of the early thermo-acido-philic and methanogenic bacteria towards the oxygen-enriched atmosphere today that supports more complex life.

A reduced version of the hypothesis has been called "influential Gaia" [ 1 ] in "Directed Evolution of the Biosphere: Biogeochemical Selection or Gaia?" by Andrei G. Lapenis, which states the biota influence certain aspects of the abiotic world, e.g. temperature and atmosphere. This is not the work of an individual but a collective of Russian scientific research that was combined into this peer reviewed publication. It states the coevolution of life and the environment through "micro-forces" [ 1 ] and biogeochemical processes. An example is how the activity of photosynthetic bacteria during Precambrian times completely modified the Earth atmosphere to turn it aerobic, and thus supports the evolution of life (in particular eukaryotic life).

Since barriers existed throughout the twentieth century between Russia and the rest of the world, it is only relatively recently that the early Russian scientists who introduced concepts overlapping the Gaia paradigm have become better known to the Western scientific community. [ 1 ] These scientists include Piotr Alekseevich Kropotkin (1842–1921) (although he spent much of his professional life outside Russia), Rafail Vasil’evich Rizpolozhensky (1862 – c. 1922), Vladimir Ivanovich Vernadsky (1863–1945), and Vladimir Alexandrovich Kostitzin (1886–1963).

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 at the end of the Archaean and the beginning of the Proterozoic periods.

Less accepted versions of the hypothesis claim 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 paradigm was an influence on the deep ecology movement. [ 2 ]

The Gaia hypothesis 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 hypothesis contends that this system as a whole, called Gaia, seeks a physical and chemical environment optimal for contemporary life. [ 3 ]

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. [ 4 ]

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 hypothesis 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. [ 5 ]

2.1. Regulation of Global Surface Temperature

Since life started on Earth, the energy provided by the Sun has increased by 25% to 30%; [ 6 ] 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. [ 7 ] This, he suggests tended to screen out ultraviolet until the formation of the ozone screen, maintaining a degree of homeostasis. However, the Snowball Earth [ 8 ] 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 pre Phanerozoic biosphere to fully self-regulate.

Processing of the greenhouse gas CO 2 , explained below, plays a critical role in the maintenance of the Earth temperature within the limits of habitability.

The CLAW hypothesis, inspired by the Gaia hypothesis, proposes a feedback loop that operates between ocean ecosystems and the Earth's climate. [ 9 ] 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.

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, [ 10 ] but he has since stated the effects will likely occur more slowly. [ 11 ]

Daisyworld simulations

In response to the criticism that the Gaia hypothesis seemingly required unrealistic group selection and cooperation between organisms, James Lovelock and Andrew Watson developed a mathematical model, Daisyworld, in which ecological competition underpinned planetary temperature regulation. [ 12 ]

Daisyworld examines the energy budget of a planet populated by two different types of plants, black daisies and white daisies, which are assumed to occupy a significant portion of the surface. The colour of the daisies influences the albedo of the planet such that black daisies absorb more light and warm the planet, while white daisies reflect more light and cool the planet. The black daisies are assumed to grow and reproduce best at a lower temperature, while the white daisies are assumed to thrive best at a higher temperature. As the temperature rises closer to the value the white daisies like, the white daisies outreproduce the black daisies, leading to a larger percentage of white surface, and more sunlight is reflected, reducing the heat input and eventually cooling the planet. Conversely, as the temperature falls, the black daisies outreproduce the white daisies, absorbing more sunlight and warming the planet. The temperature will thus converge to the value at which the reproductive rates of the plants are equal.

Lovelock and Watson showed that, over a limited range of conditions, this negative feedback due to competition can stabilize the planet's temperature at a value which supports life, if the energy output of the Sun changes, while a planet without life would show wide temperature changes. The percentage of white and black daisies will continually change to keep the temperature at the value at which the plants' reproductive rates are equal, allowing both life forms to thrive.

It has been suggested that the results were predictable because Lovelock and Watson selected examples that produced the responses they desired. [ 13 ]

2.2. Regulation of Oceanic Salinity

Ocean salinity has been constant at about 3.5% for a very long time. [ 14 ] 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 [ 15 ] 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. [ 14 ]

In the biogeochemical processes of Earth, sources and sinks are the movement of elements. The composition of salt ions within our oceans and seas is: sodium (Na + ), chlorine (Cl − ), sulfate (SO 4 2− ), magnesium (Mg 2+ ), calcium (Ca 2+ ) and potassium (K + ). The elements that comprise salinity do not readily change and are a conservative property of seawater. [ 14 ] There are many mechanisms that change salinity from a particulate form to a dissolved form and back. Considering the metallic composition of iron sources across a multifaceted grid of thermomagnetic design, not only would the movement of elements hypothetically help restructure the movement of ions, electrons, and the like, but would also potentially and inexplicably assist in balancing the magnetic bodies of the Earth's geomagnetic field. The known sources of sodium i.e. salts are when weathering, erosion, and dissolution of rocks are transported into rivers and deposited into the oceans.

The Mediterranean Sea as being Gaia's kidney is found (here) by Kenneth J. Hsue, a correspondence author in 2001. Hsue suggests the "desiccation" of the Mediterranean is evidence of a functioning Gaia "kidney". In this and earlier suggested cases, it is plate movements and physics, not biology, which performs the regulation. Earlier "kidney functions" were performed during the "deposition of the Cretaceous (South Atlantic), Jurassic (Gulf of Mexico), Permo-Triassic (Europe), Devonian ( Canada ), and Cambrian/Precambrian (Gondwana) saline giants." [ 16 ]

2.3. Regulation of Oxygen in the Atmosphere

The Gaia theorem states that the Earth's atmospheric composition is kept at a dynamically steady state by the presence of life. [ 17 ] The atmospheric composition provides 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 stability of the atmosphere in Earth is not a consequence of chemical equilibrium. Oxygen is a reactive compound, and should eventually combine with gases and minerals of the Earth's atmosphere and crust. Oxygen only began to persist in the atmosphere in small quantities about 50 million years before the start of the Great Oxygenation Event. [ 18 ] Since the start of the Cambrian period, atmospheric oxygen concentrations have fluctuated between 15% and 35% of atmospheric volume. Cite error: Closing </ref> missing for <ref> tag 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. Some arrive at the bottom of the oceans where plate tectonics and heat and/or pressure eventually convert them to deposits of chalk and limestone. Much of the falling dead shells, however, re-dissolve into the ocean below the carbon compensation depth.

One of these organisms is Emiliania huxleyi , an abundant coccolithophore algae which may have a role in the formation of clouds. [ 19 ] CO 2 excess is compensated by an increase of coccolithophorid life, increasing the amount of CO 2 locked in the ocean floor. Coccolithophorids, if the CLAW Hypothesis turns out to be supported (see "Regulation of Global Surface Temperature" above), could help increase the cloud cover, hence control the surface temperature, help cool the whole planet and favor precipitation necessary for terrestrial plants. Lately the atmospheric CO 2 concentration has increased and there is some evidence that concentrations of ocean algal blooms are also increasing. [ 20 ]

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 CO 2 levels rise in the atmosphere the temperature increases and plants grow. This growth brings higher consumption of CO 2 by the plants, who process it into the soil, removing it from the atmosphere.

3.1. Precedents

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" (from Ge = Earth, and Aia = PIE grandmother), 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. [ 21 ] 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. [ 22 ] Later, the naturalist and explorer Alexander von Humboldt recognized the coevolution of living organisms, climate, and Earth's crust. [ 22 ] In the twentieth century, Vladimir Vernadsky formulated a theory of Earth's development that is now one of the foundations of ecology. Vernadsky was a Ukrainian geochemist and 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. [ 23 ] His visionary pronouncements were not widely accepted in the West, and some decades later 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.

Another influence for the Gaia hypothesis 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, through the Overview Effect an early symbol for the global ecology movement. [ 24 ]

3.2. Formulation of the Hypothesis

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. [ 25 ] [ 26 ] 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. [ 27 ] 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 [ 28 ] and 1974, [ 29 ] followed by a popularizing 1979 book Gaia: A new look at life on Earth . An article in the New Scientist of February 6, 1975, [ 30 ] 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, [ 31 ] 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 hypothesis. [ 32 ]

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. [ 33 ] The American biologist had also awakened criticism from the scientific community with her advocacy of the 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'.

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 [ 34 ] and provided a number of useful predictions. [ 35 ]

3.3. 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. [ 36 ] 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. [ 37 ]

3.4. Second Gaia Conference

In 1988, climatologist Stephen Schneider organised a conference of the American Geophysical Union. The first Chapman Conference on Gaia, [ 38 ] was held in San Diego, California on March 7, 1988.

During the "philosophical foundations" session of the conference, David Abram spoke on the influence of metaphor in science, and of the Gaia hypothesis 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 hypothesis 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. [ 39 ]

Lovelock and other Gaia-supporting scientists, however, did attempt to disprove the claim that the hypothesis 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. [ 12 ] 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". [ 40 ]

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 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.

3.5. Third Gaia Conference

By the time of the 2nd Chapman Conference on the Gaia Hypothesis, held at Valencia, Spain, on 23 June 2000, [ 41 ] the situation had changed significantly. Rather than a discussion of the Gaian teleological views, or "types" of Gaia hypotheses, 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: [ 42 ]

• "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 collaborated with 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 symbiotic 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. 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 and vibration-based beings and organisms. The resulting co-evolving dynamical process eventually leads to the convergence of equilibrium and optimal conditions", but would also require progress of truth and understanding in a lens that could be argued was put on hiatus while the species was proliferating the needs of Economic manipulation and environmental degradation while losing sight of the maturing nature of the needs of many. (12:22 10.29.2020)

3.6. Fourth Gaia Conference

A fourth international conference on the Gaia hypothesis, sponsored by the Northern Virginia Regional Park Authority and others, was held in October 2006 at the Arlington, VA campus of George Mason University. [ 43 ]

Martin Ogle, Chief Naturalist, for NVRPA, and long-time Gaia hypothesis 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 hypothesis, 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.

4. Criticism

After initially receiving little attention from scientists (from 1969 until 1977), thereafter for a period the initial Gaia hypothesis was criticized by a number of scientists, including Ford Doolittle, [ 44 ] Richard Dawkins [ 45 ] and Stephen Jay Gould. [ 38 ] Lovelock has said that because his hypothesis is named after a Greek goddess, and championed by many non-scientists, [ 31 ] the Gaia hypothesis was interpreted as a neo-Pagan religion. Many scientists in particular also criticized the approach taken in his popular book Gaia, a New Look at Life on Earth for being teleological—a belief that things are purposeful and 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 criticized Gaia as being "a metaphor, not a mechanism." [ 46 ] He wanted to know the actual mechanisms by which self-regulating homeostasis was achieved. In his defense of Gaia, David Abram argues that Gould overlooked the fact that "mechanism", itself, is a metaphor — albeit an exceedingly common and often unrecognized metaphor — one which leads us to consider natural and living systems as though they were machines organized and built from outside (rather than as autopoietic or self-organizing phenomena). Mechanical metaphors, according to Abram, lead us to overlook the active or agent quality of living entities, while the organismic metaphors of the Gaia hypothesis accentuate the active agency of both the biota and the biosphere as a whole. [ 47 ] [ 48 ] With regard to causality in Gaia, Lovelock argues 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, and that specific hostility is reserved for his own hypothesis for other reasons. [ 49 ]

Aside from clarifying his language and understanding of what is meant by a life form, Lovelock himself ascribes most of the criticism to a lack of understanding of non-linear mathematics by his critics, and a linearizing form of greedy reductionism in which all events have to be immediately ascribed to specific causes before the fact. He also states that most of his critics are biologists but that his hypothesis includes experiments in fields outside biology, and that some self-regulating phenomena may not be mathematically explainable. [ 49 ]

4.1. 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 the early 1980s, W. Ford Doolittle and Richard Dawkins separately argued against this aspect of Gaia. Doolittle 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. [ 44 ] Dawkins meanwhile stated that for organisms to act in concert would require foresight and planning, which is contrary to the current scientific understanding of evolution. [ 45 ] Like Doolittle, he also rejected the possibility that feedback loops could stabilize the system.

Lynn Margulis, a microbiologist who collaborated with Lovelock in supporting the Gaia hypothesis, 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 those set points change with time. [ 50 ]

Evolutionary biologist W. D. Hamilton called the concept of Gaia Copernican, adding that it would take another Newton to explain how Gaian self-regulation takes place through Darwinian natural selection. [ 21 ] More recently Ford Doolittle building on his and Inkpen's ITSNTS (It's The Song Not The Singer) proposal [ 51 ] proposed that differential persistence can play a similar role to differential reproduction in evolution by natural selections, thereby providing a possible reconciliation between the theory of natural selection and the Gaia hypothesis. [ 52 ]

4.2. Criticism in the 21st Century

The Gaia hypothesis continues to be broadly skeptically received by the scientific community. For instance, arguments both for and against it were laid out in the journal Climatic Change in 2002 and 2003. A significant argument raised against it are the many examples where life has had a detrimental or destabilising effect on the environment rather than acting to regulate it. [ 53 ] [ 54 ] Several recent books have criticised the Gaia hypothesis, expressing views ranging from "... the Gaia hypothesis lacks unambiguous observational support and has significant theoretical difficulties" [ 55 ] to "Suspended uncomfortably between tainted metaphor, fact, and false science, I prefer to leave Gaia firmly in the background" [ 56 ] to "The Gaia hypothesis is supported neither by evolutionary theory nor by the empirical evidence of the geological record". [ 57 ] The CLAW hypothesis, [ 9 ] initially suggested as a potential example of direct Gaian feedback, has subsequently been found to be less credible as understanding of cloud condensation nuclei has improved. [ 58 ] In 2009 the Medea hypothesis was proposed: that life has highly detrimental (biocidal) impacts on planetary conditions, in direct opposition to the Gaia hypothesis. [ 59 ]

In a 2013 book-length evaluation of the Gaia hypothesis considering modern evidence from across the various relevant disciplines, Toby Tyrrell concluded that: "I believe Gaia is a dead end*. Its study has, however, generated many new and thought provoking questions. While rejecting Gaia, we can at the same time appreciate Lovelock's originality and breadth of vision, and recognize that his audacious concept has helped to stimulate many new ideas about the Earth, and to champion a holistic approach to studying it". [ 60 ] Elsewhere he presents his conclusion "The Gaia hypothesis is not an accurate picture of how our world works". [ 61 ] This statement needs to be understood as referring to the "strong" and "moderate" forms of Gaia—that the biota obeys a principle that works to make Earth optimal (strength 5) or favourable for life (strength 4) or that it works as a homeostatic mechanism (strength 3). The latter is the "weakest" form of Gaia that Lovelock has advocated. Tyrrell rejects it. However, he finds that the two weaker forms of Gaia—Coeveolutionary Gaia and Influential Gaia, which assert that there are close links between the evolution of life and the environment and that biology affects the physical and chemical environment—are both credible, but that it is not useful to use the term "Gaia" in this sense and that those two forms were already accepted and explained by the processes of natural selection and adaptation. [ 62 ]

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• Cockell, Charles; Corfield, Richard; Dise, Nancy; Edwards, Neil; Harris, Nigel (2008). An Introduction to the Earth-Life System. Cambridge (UK): Cambridge University Press. ISBN 9780521729536. http://www.cambridge.org/us/academic/subjects/earth-and-environmental-science/palaeontology-and-life-history/introduction-earth-life-system. 
• Quinn, P.K.; Bates, T.S. (2011), "The case against climate regulation via oceanic phytoplankton sulphur emissions", Nature 480 (7375): 51–56, doi:10.1038/nature10580, PMID 22129724, Bibcode: 2011Natur.480...51Q, https://zenodo.org/record/1233319 
• Peter Ward (2009), The Medea Hypothesis: Is Life on Earth Ultimately Self-Destruction?, ISBN:0-691-13075-2
• Tyrrell, Toby (2013), On Gaia: A Critical Investigation of the Relationship between Life and Earth, Princeton: Princeton University Press, p. 209, ISBN 9780691121581, http://press.princeton.edu/titles/9959.html 
• Tyrrell, Toby (26 October 2013), "Gaia: the verdict is…", New Scientist 220 (2940): 30–31, doi:10.1016/s0262-4079(13)62532-4  https://dx.doi.org/10.1016%2Fs0262-4079%2813%2962532-4
• Tyrrell, Toby (2013), On Gaia: A Critical Investigation of the Relationship between Life and Earth, Princeton: Princeton University Press, p. 208, ISBN 9780691121581, http://press.princeton.edu/titles/9959.html 

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Gaia hypothesis

The Gaia (pronounced GAY-ah) hypothesis is the idea that Earth is a living organism and can regulate its own environment. This idea argues that Earth is able to maintain conditions that are favorable for life to survive on it, and that it is the living things on Earth that give the planet this ability.

Mother Earth

The idea that Earth and its atmosphere are some sort of "superorganism" was actually first proposed by Scottish geologist (a person specializing in the study of Earth) James Hutton (1726–1797), although this was not one of his more accepted and popular ideas. As a result, no one really pursued this notion until some 200 years later, when the English chemist James Lovelock (1919– ) put forth a similar idea in his 1979 book, Gaia: A New Look at Life on Earth. Gaia is the name of the Greek goddess of Earth and mother of the Titans. In modern times, the name has come to symbolize "Earth Mother" or "Living Earth." In this book, Lovelock proposed that Earth's biosphere (all the parts of Earth that make up the living world) acts as a single living system that if left alone, can regulate itself.

As to the name Gaia, the story goes that Lovelock was walking in the countryside surrounding his home in Wilshire, England, and met his neighbor, English novelist William Golding (1911–1993), author of Lord of the Flies and several other books. Telling Golding of his new theory, he then asked his advice about choosing a suitable name for it, and the result of this meeting was that the term "Gaia" was chosen because of its real connection to the Greek goddess who pulled the living world together out of chaos or complete disorder.

Origin of Earth's atmosphere

Lovelock arrived at this hypothesis by studying Earth's neighboring planets, Mars and Venus. Suggesting that chemistry and physics seemed to argue that these barren and hostile planets should have an atmosphere just like that of Earth, Lovelock stated that Earth's atmosphere is different because it has life on it. Both Mars and Venus have an atmosphere with about 95 percent carbon dioxide, while Earth's is about 79 percent nitrogen and 21 percent oxygen. He explained this dramatic difference by saying that Earth's atmosphere was probably very much like that of its neighbors at first, and that it was a world with hardly any life on it. The only form that did exist was what many consider to be the first forms of life—anaerobic (pronounced ANN-ay-roe-bik) bacteria that lived in the ocean. This type of bacteria cannot live in an oxygen environment, and its only job is to convert nitrates to nitrogen gas. This accounts for the beginnings of a nitrogen build-up in Earth's atmosphere.

Words to Know

Biosphere: The sum total of all lifeforms on Earth and the interaction among those lifeforms.

Feedback: Information that tells a system what the results of its actions are.

Homeostasis: State of being in balance; the tendency of an organism to maintain constant internal conditions despite large changes in the external environment.

Photosynthesis: Chemical process by which plants containing chlorophyll use sunlight to manufacture their own food by converting carbon dioxide and water to carbohydrates, releasing oxygen as a by-product.

Symbiosis: A pattern in which two or more organisms live in close connection with each other, often to the benefit of both or all organisms.

The oxygen essential to life as we know it did not start to accumulate in the atmosphere until organisms that were capable of photosynthesis evolved. Photosynthesis is the process that some algae and all plants use to convert chemically the Sun's light into food. This process uses carbon dioxide and water to make energy-packed glucose, and it gives off oxygen as a by-product. These very first photosynthesizers were a blue-green algae called cyanobacteria (pronounced SIGH-uh-no-bak-teer-eea) that live in water. Eventually, these organisms produced so much oxygen that they put the older anaerobic bacteria out of business. As a result, the only place that anaerobic bacteria could survive was on the deep-sea floor (as well as in heavily water-logged soil and in our own intestines). Love-lock's basic point was that the existence of life (bacteria) eventually made Earth a very different place by giving it an atmosphere.

Lovelock eventually went beyond the notion that life can change the environment and proposed the controversial Gaia hypothesis. He said that Gaia is the "Living Earth" and that Earth itself should be viewed as being alive. Like any living thing, it always strives to maintain constant or stable conditions for itself, called homeostasis (pronounced hoe-mee-o-STAY-sis). In the Gaia hypothesis, it is the presence and activities of life that keep Earth in homeostasis and allow it to regulate its systems and maintain steady-state conditions.

Cooperation over competition

Lovelock was supported in his hypothesis by American microbiologist Lynn Margulis (1918– ) who became his principal collaborator. Margulis not only provided support, but she brought her own scientific ability and achievements to the Gaia hypothesis. In her 1981 book, Symbiosis in Cell Evolution , Margulis had put forth the then-unheard of theory that life as we know it today evolved more from cooperation than from competition. She argued that the cellular ancestors of today's plants and animals were groups of primitive, formless bacteria cells called prokaryotes (pronounced pro-KAR-ee-oats). She stated that these simplest of bacteria formed symbiotic relationships—relationships that benefitted both organisms—which eventually led to the evolution of new lifeforms. Her theory is called endosymbiosis (pronounced en-doe-sim-bye-O-sis) and is based on the fact that bacteria routinely take and transfer bits of genetic material from each other.

Margulis then argued that simple bacteria eventually evolved into more complex eukaryotic (pronounced you-kar-ee-AH-tik) cells or cells with a nucleus. These types of cells form the basic structure of plants and animals. Her then-radical but now-accepted idea was that life evolved more out of cooperation (which is what symbiosis is all about) than it did out of competition (in which only the strong survive and reproduce). The simple prokaryotes did this by getting together and forming symbiotic groups or systems that increased their chances of survival. According to Margulis then, symbiosis, or the way different organisms adapt to living together to the benefit of each, was the major mechanism for change on Earth.

Most scientists now agree with her thesis that oxygen-using bacteria joined together with fermenting bacteria to form the basis of a type of new cell that eventually evolved into complex eukaryotes. For the Gaia hypothesis, the Margulis concept of symbiosis has proven to be a useful explanatory tool. Since it explains the origin and the evolution of life on Earth (by stating that symbiosis is the mechanism of change), it applies also to what continues to happen as the process of evolution goes on and on.

Gaia explained

The main idea behind the Gaia hypothesis can be both simple and complex. Often, several similar examples or analogies concerning the bodies of living organisms are used to make the Gaia concept easier to understand. One of these states that we could visualize Earth's rain forests as the lungs of the planet since they exchange oxygen and carbon dioxide. Earth's atmosphere could be thought of as its respiratory system, and its streams of moving water and larger rivers like its circulatory system, since they bring in clean water and flush out the system. Some say that the planet actually "breathes" because it contracts and expands with the Moon's gravitational pull, and the seasonal changes we all experience are said to reflect our own rhythmic bodily cycles.

Many of these analogies are useful in trying to explain the general idea behind the Gaia hypothesis, although they should not be taken literally. Lovelock, however, has stated that Earth is very much like the human body in that both can be viewed as a system of interacting components. He argues that just as our bodies are made up of billions of cells working together as a single living being, so too are the billions of different lifeforms on Earth working together (although unconsciously) to form a single, living "superorganism." Further, just as the processes or physiology of our bodies has its major systems (such as the nervous system, circulatory system, respiratory system, etc.), so, says Lovelock, Earth has its own "geophysiology." This geophysiology is made up of four main components: atmosphere (air), biosphere (all lifeforms), geosphere (soil and rock), and hydrosphere (water). Finally, just as our own physiological health depends on all of our systems being in good working condition and, above all, working together well, so, too, does Earth's geophysiology depend on its systems working in harmony.

Life is the regulating mechanism

Lovelock claims that all of the living things on Earth provide it with this necessary harmony. He states that these living things, altogether, control the physical and chemical conditions of the environment, and therefore it is life itself that provides the feedback that is so necessary to regulating something. Feedback mechanisms can detect and reverse any unwanted changes. A typical example of feedback is the thermostat in most homes. We set it to maintain a comfortable indoor temperature, usually somewhere in the range between 65°F (18°C) and 70°F (21°C). The thermostat is designed so that when the temperature falls below a certain setting, the furnace is turned on and begins to heat the house. When that temperature is reached and the thermostat senses it, the furnace is switched off. Our own bodies have several of these feedback mechanisms, all of which are geared to maintaining conditions within a certain proper and balanced range.

For Earth's critical balance, Lovelock says that it is the biosphere, or all of life on Earth, that functions as our thermostat or regulator. He says that the atmosphere, the oceans, the climate, and even the crust of Earth are regulated at a state that is comfortable for life because of the behavior of living organisms . This is the revolutionary lesson that the Gaia hypothesis wants to teach. It says that all of Earth's major components, such as the amount of oxygen and carbon dioxide in the atmosphere, the

saltiness of the oceans, and the temperature of our surface is regulated or kept in proper balance by the activities of the life it supports. He also states that this feedback system is self-regulating and that it happens automatically. As evidence that, if left alone, Earth can regulate itself, he asserts that it is the activity of living organisms that maintain the delicate balance between atmospheric carbon dioxide and oxygen. In a way, Love-lock argues that it is life itself that maintains the conditions favorable for the continuation of life. For example, he contends that it is no accident that the level of oxygen is kept remarkably constant in the atmosphere at 21 percent. Lovelock further offers several examples of cycles in the environment that work to keep things on an even keel.

Lovelock also warns that since Earth has the natural capacity to keep things in a stable range, human tampering with Earth's environmental balancing mechanisms places everyone at great risk. While environmentalists insist that human activity (such as industrial policies that result in harming Earth's ozone layer) is upsetting Earth's ability to regulate itself, others who feel differently argue that Earth can continue to survive very well no matter what humans do exactly because of its built-in adaptability.

Earth as seen from space

An important aspect about the Gaia hypothesis is that it offers scientists a new model to consider. Most agree that such a different type of model was probably not possible to consider seriously until humans went into space. However, once people could travel beyond the atmosphere of Earth and put enough distance between them and their planet, then they could view their home from an extra-terrestrial viewpoint. No doubt that the 1960s photographs of the blue, green, and white ball of life floating in the total darkness of outer space made both scientists and the public think of their home planet a little differently than they ever had before. These pictures of Earth must have brought to mind the notion that it resembled a single organism.

Although the Gaia hypothesis is still very controversial and has not been established scientifically (by being tested and proven quantitatively), it has already shown us the valuable notion of just how interdependent everything is on Earth. We now recognize that Earth's biological, physical, and chemical components or major parts regularly interact with and mutually affect one another, whether by accident or on purpose. Finally, it places great emphasis on what promises to be the planet's greatest future problem—the quality of Earth's environment and the role humans will play in Earth's destiny.

[ See also Biosphere ; Ecosystem ]

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The Gaia Hypothesis: What Does It Mean for Life on Earth?

By Leonie Barghorn Categories: Environment & Nature May 11, 2021, 9:07 AM

Some consider the Gaia hypothesis mere spiritual speculation – for others, it’s key to understanding life on Earth. Here’s the truth about the Gaia theory.

In Greek mythology, Gaia was one of the oldest goddesses of all, the personification of the Earth itself. Thousands of years later, James Lovelock – a biochemist whose environmental work began in the 1970s – had a groundbreaking idea. He proposed we think of the Earth as a living being, a vast superorganism. Lovelock and his colleague, the microbiologist Lynn Margulis, named this theory the Gaia hypothesis .

The Gaia Hypothesis: The Scientific Evidence

The two scientists established that certain parameters on Earth have remained stable for hundreds of millions of years, including:

• oxygen levels in the atmosphere
• the salinity of the oceans
• the surface temperature of the Earth

Lovelock and Margulis concluded that, to maintain this balance, all of Earth’s organic and inorganic constituents must be linked. This interconnectedness is sufficient, they argued, to consider the Earth itself to be a self-regulating organism . Its lifeforms don’t just adapt to the prevailing conditions on Earth – they actually drive and determine those conditions. The regulatory mechanisms involved are similar to those at work in the human body, for example.

Welcome to Daisyworld

To demonstrate the scientific validity of the Gaia hypothesis, Lovelock developed a computer model which he called Daisyworld. The Daisyworld model shows, for example, how a planet’s inhabitants can work to stabilize its temperature, even as the intensity of its sun steadily increases.

Daisyworld is a planet similar to Earth, with a heat source – a sun – just like ours. However, Daisyworld has only two lifeforms: black daisies and white daisies . Initially, the sun’s light is faint, and only the black daisies can survive. They are better suited to lower temperatures, because their darker color reflects less light, and they are able to absorb more sunlight.

As the black daisies absorb the sunlight, the planet itself slowly heats up. As it gets warmer, however, the population of white daisies increases. Their lighter color gives them an advantage at higher temperatures because it doesn’t absorb unnecessary heat. White daisies, consequently, tend to cool the planet, because they reflect more sunlight back into space .

The resulting negative feedback loop between the two species of daisies means that the temperature on the planet remains relatively constant, even as the intensity of the sun increases. Daisyworld is thus an example of a self-regulating system . This simple planet can sustain a balanced biosphere for eons, until the sun itself eventually becomes too hot.

How Our Biosphere Influences the Climate

Of course, our Earth is far more complex than Daisyworld. There are countless real-life feedback loops which show how the biosphere affects the climate:

• Warmer temperatures encourage the growth of algae. They produce sulfur compounds that help clouds to form in the Earth’s atmosphere. The clouds reflect some of the incoming sunlight and, in turn, cool the Earth.
• Oceans receive a constant supply of minerals from rivers and hydrothermal vents on the ocean floor. Nevertheless, the salinity of the oceans remains constant. This is due, on the one hand, to the fact that minerals are constantly deposited back onto the ocean floor. However, there are also microorganisms which actively extract minerals from the seawater .
• Through photosynthesis, plants consume CO 2 , which helps to regulate the atmosphere’s temperature via the greenhouse effect . If CO 2 levels – and thus temperatures – increase, more plants are able to grow in regions further from the equator. However, more plants also consume more CO 2 . Consequently, CO 2 levels in the atmosphere decrease, and temperatures fall again.

However, these examples are only a tiny fraction of the incredibly complex ecological relationships on our planet. Is there enough evidence to confirm the Gaia hypothesis?

The Gaia Hypothesis: Criticism and Consensus

The Gaia hypothesis continues to be popular in more spiritual circles, but there is little scientific consensus on its validity. One reason is simply that the Earth is so complex, and the Gaia theory so broad, that it’s virtually impossible to definitively prove or disprove the hypothesis.

This is the main reason that criticism of the concept is so widespread. The scientific arguments against the Gaia hypothesis are well summed-up in Toby Tyrell’s book On Gaia: A Critical Investigation of the Relationship between Life and Earth . Examining evidence from fields such as geology, biology, and oceanography (to name but a few!), he concludes that the feedback loops which are undeniably present in our biosphere are not sufficient to reliably and eternally stabilize habitable conditions on Earth. “My aim was to determine whether the Gaia hypothesis is a credible explanation of how life and environment interact on Earth – I found it is not.” The book – which is actually a readable, accessible introduction to the evolution of life on Earth – instead proposes co-evolution as a more valid theory. It is available in many book shops and on Amazon **.

Nevertheless, the Gaia hypothesis is undeniably useful as a metaphor to raise awareness for the interconnectedness of all lifeforms. It may be a stretch, scientifically speaking, to describe the Earth as a superorganism like a colony of bees . But we are only beginning to realize the true importance of biodiversity for the climate and for humanity’s own survival. From this perspective, the Gaia hypothesis is more relevant than ever .

The Gaia Theory Is Not an Excuse for Climate Change Denial

In the face of epochal, life-threatening climate change, we might find the Gaia hypothesis reassuring. If the Earth is self-regulating, like a living thing, surely everything will balance out eventually? But even if the Gaia theory is true: Living things can get sick . James Lovelock himself admitted this over ten years ago. In his 2009 book The Vanishing Face of Gaia: A Final Warning , he proposes that “Gaia’s illness could be called polyanthroponemia, where humans overpopulate until they do more harm than good.” This seems less a theory and more a sad statement of fact.

As we all know, humans continue to disrupt the Earth’s own regulatory mechanisms. By clearing forests , for instance, we prevent them from compensating for increasing atmospheric CO 2 levels. There are numerous ecological tipping points in the Earth’s biosphere. When these are exceeded, irreversible changes are set in motion. For example, as the climate warms, permafrost increasingly thaws, releasing CO 2 and methane. The resulting greenhouse effect warms the Earth further, so more permafrost thaws, and the cycle continues.

And even if the Earth as a whole is self-sustaining, that doesn’t necessarily mean that every single species will survive. Some scientists are already warning of the imminent extinction of mankind . We will have to change ourselves to save the planet – because the planet isn’t going to save us.

• Eco-Anxiety: Climate Change Stress and How to Cope
• What Would Happen if Bees Went Extinct? These Ten Things Would Disappear
• 8 Things You Can Do to Save the Ocean

This article was adapted and translated from German to English by Will Tayler . You can read the original here: Gaia-Hypothese: Einfach und verständlich für Laien erklärt

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Definition of Gaia

Word history.

after Greek Gaîa , a primordial earth goddess in Greek myth, literally, "earth" — more at geo-

1972, in the meaning defined above

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“Gaia.” Merriam-Webster.com Dictionary , Merriam-Webster, https://www.merriam-webster.com/dictionary/Gaia. Accessed 16 Oct. 2024.

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The Gaia Hypothesis is a scientific theory proposed by James Lovelock, suggesting that the Earth and its biological systems behave as a single, self-regulating entity. This concept implies that life interacts with the Earth's environment to maintain conditions suitable for life, drawing connections to ancient beliefs about the Earth as a living organism and its reflection in various mythologies and religions.

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5 Must Know Facts For Your Next Test

• The Gaia Hypothesis suggests that biological processes regulate the Earth's environment, maintaining conditions for life through feedback mechanisms.
• Lovelock proposed this idea in the 1970s, highlighting how life interacts with geological processes to create a balanced ecosystem.
• The hypothesis encourages viewing Earth not just as a collection of individual organisms but as an integrated system with interdependent relationships.
• The concept resonates with ancient mythologies where Earth is often viewed as a deity or a living organism, influencing how cultures perceive nature.
• The Gaia Hypothesis has sparked discussions in both scientific communities and environmental movements about sustainability and ecological balance.

Review Questions

• The Gaia Hypothesis reflects ancient beliefs by presenting the Earth as a living entity that sustains life through interconnected systems. Many cultures viewed the Earth as a nurturing mother or goddess, suggesting an intrinsic value and respect for nature. This connection highlights the importance of environmental stewardship, emphasizing that human actions impact this delicate balance and calling for a more responsible approach to how we interact with our planet.
• The Gaia Hypothesis challenges traditional scientific perspectives by proposing that life actively shapes its environment rather than being merely affected by it. This idea shifts focus from isolated species or events to broader interactions within ecosystems. By framing Earth as a self-regulating system, it invites a reevaluation of ecological studies and emphasizes the need for holistic approaches when addressing environmental issues.
• The Gaia Hypothesis has significantly influenced modern environmental movements by promoting an understanding of Earth's interconnectedness. It has inspired advocacy for sustainable practices, emphasizing that human health is tied to the health of our planet. In policy-making, this perspective encourages policies that prioritize ecological balance and sustainability, challenging short-term economic gains in favor of long-term planetary well-being. This holistic approach aims to foster a sense of responsibility towards preserving our environment for future generations.

Related terms

Anthropocene : The current geological epoch characterized by significant human impact on Earth's geology and ecosystems, often linked to discussions about climate change and environmental degradation.

Holism : An approach in science and philosophy that emphasizes the importance of understanding systems as wholes rather than merely the sum of their parts, relating to how the Gaia Hypothesis views Earth as an interconnected system.

Mother Earth : A common personification of the Earth as a nurturing figure, often found in various mythologies and cultures, reflecting the concept of the Earth as a living entity akin to the ideas presented in the Gaia Hypothesis.

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• Art and Climate Change
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• Exoplanetary Science
• Introduction to Environmental Systems
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• Plate Tectonics
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