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Understanding planetary habitability is partly an extrapolation of the Earth's conditions, as it is the only planet currently known to harbor life. The release of this image had an effect in leading to the proposition that the Earth was alive.The Gaia hypothesis is an ecological theory that proposes that the living matter of planet Earth functions like a single organism. An early recognition of some of the core assumptions of the Gaia hypothesis was given in the book Lives of a Cell by Lewis Thomas. It was first scientifically formulated in the 1960s by the independent research scientist James Lovelock, as a consequence of his work for NASA on methods of detecting life on Mars. He wrote an article in the science journal Nature, before publishing the concept in the 1979 book Gaia: A new look at life on Earth. He named this self-regulating living system after the Greek goddess Gaia, using a suggestion from the novelist William Golding. The Gaia Hypothesis has since been supported by a number of scientific experiments and provided a number of useful predictions so is properly referred to as the Gaia Theory.
Since 1971, the noted microbiologist Lynn Margulis has been Lovelock's most important collaborator in developing Gaian concepts (Turney 2003). Encouraged by Margulis, the theory was first publicly mentioned in the article by Lovelock, "Gaia as seen through the atmosphere" in the Journal, Atmospheric Environment 6 (1972) pp.579-580.
Until 1975 it was almost totally ignored. An article in the New Scientist of February 15th, 1975, and a popular book length version of the theory, published as The Quest for Gaia, began to attract scientific and critical attention to the hypothesis. The theory was then attacked by many mainstream biologists. Championed by certain environmentalists and climate scientists, it was vociferously rejected by many others, both within scientific circles and outside of them. . Lovelock draws comparison with the resistance to the introduction of the idea of plate tectonics within geology, noting that it took about 30 years before it became universally accepted as true.
Today the Gaia theory is more commonly referred to as earth system science, and is a class of scientific models of the geo-biosphere in which life as a whole fosters and maintains suitable conditions for itself by helping to create an environment on Earth suitable for its continuity.
Gaia "theories" have non-technical predecessors in the ideas of many cultures. Today, "Gaia theory" is sometimes used among scientists on the basis that the earlier Gaia hypothesis has withstood rigorous scientific testing. It is also used by non-scientists to refer to hypotheses of a self-regulating Earth that are non-technical but take inspiration from scientific models. At the second Chapman Conference of the American Geophysical Union, at Valencia in Spain, for instance, Lynn Margulis in her closing address "Modes of Confirmation of the Gaia Hypothesis" conceded that despite being elevated to "Gaia theory" in the 1980s, there was still confusion about what Gaia was in reality. Among some scientists, "Gaia" still carries connotations of lack of scientific rigor and quasi-mystical thinking about the planet Earth. Lovelock's own reframing of the hypothesis as "Geophysiology" and the growing acceptance of "Earth system science" has silenced many of these critics.
Lovelock's initial hypothesis
Lovelock defined Gaia as:
a complex entity involving the Earth's biosphere, atmosphere, oceans, and soil; the totality constituting a feedback or cybernetic system which seeks an optimal physical and chemical environment for life on this planet.
His initial hypothesis was that the biomass modifies the conditions on the planet to make conditions on the planet more hospitable – the Gaia Hypothesis properly defined this "hospitality" as a full homeostasis. Lovelock's initial hypothesis, accused of being teleological by his critics, was that the atmosphere is kept in homeostasis by and for the biosphere.
Lovelock suggested that life on Earth provides a cybernetic, homeostatic feedback system operated automatically and unconsciously by the biota, leading to broad stabilization of global temperature and chemical composition.
With his initial hypothesis, Lovelock claimed the existence of a global control system of surface temperature, atmosphere composition and ocean salinity. His arguments were:
The global surface temperature of the Earth has remained constant, despite an increase in the energy provided by the Sun.
Atmospheric composition remains constant, even though it should be unstable.
Ocean salinity is constant.
Since life started on Earth, the energy provided by the Sun has increased by 25% to 30%; however the surface temperature of the planet has remained remarkably constant when measured on a global scale. Furthermore, he argued, the atmospheric composition of the Earth is constant. The Earth's atmosphere currently consists of 79% nitrogen, 20.7% oxygen and 0.03% carbon dioxide. Oxygen is the second most reactive element after fluorine, and should combine with gases and minerals of the Earth's atomosphere and crust. Traces of methane (at an amount of 100,000 tonnes produced per annum), should not exist, as methane is combustible in an oxygen atmosphere. This composition should be unstable, and its stability can only have been maintained with removal or production by living organisms.
Ocean salinity has been constant at about 3.4% for a very long time. Salinity stability is important as most cells require a rather constant salinity and do not generally tolerate values above 5%. Ocean salinity constancy was a long-standing mystery, because river salts should have raised the ocean salinity much higher than observed. Recently it was suggested that salinity may also be strongly influenced by seawater circulation through hot basaltic rocks, and emerging as hot water vents on ocean spreading ridges. However, the composition of sea water is far from equilibrium, and it is difficult to explain this fact without the influence of organic processes.
The only significant natural source of atmospheric carbon dioxide (CO2) is volcanic activity, while the only significant removal is through the precipitation of carbonate rocks. In water, CO2 is dissolved as a "carbonic acid," which may be combined with dissolved calcium to form solid calcium carbonate (limestone). Both precipitation and solution are influenced by the bacteria and plant roots in soils, where they improve gaseous circulation, or in coral reefs, where calcium carbonate is deposited as a solid on the sea floor. Calcium carbonate can also be washed from continents to the sea where it is used by living organisms to manufacture carbonaceous tests and shells. Once dead, the living organisms' shells fall to the bottom of the oceans where they generate deposits of chalk and limestone. Part of the organisms with carboneous shells are the coccolithophores (algae), which also happen to participate in the formation of clouds. When they die, they release a sulfurous gas (DMS), (CH3)2S, which act as particles on which water vapor condenses to make clouds.
Lovelock sees this as one of the complex processes that maintain conditions suitable for life. The volcanoes produce CO2 in the atmosphere, CO2 participates in rock weathering as carbonic acid, itself accelerated by temperature and soil life, the dissolved CO2 is then used by the algae and released on the ocean floor. CO2 excess can be compensated by an increase of coccolithophoride life, increasing the amount of CO2 locked in the ocean floor. Coccolithophorides increase the cloud cover, hence control the surface temperature, help cool the whole planet and favor precipitations which are necessary for terrestrial plants. For Lovelock and other Gaia scientists like Stephan Harding, coccolithophorides are one stage in a regulatory feedback loop. Lately the atmospheric CO2 concentration has increased and there is some evidence that concentrations of ocean algal blooms are also increasing.
 Controversial concepts
Lovelock, especially in his older texts, has often indulged in language that has later caused fiery debates. For instance many attacked his statement in the first paragraph of his first Gaia book (1979), that "the quest for Gaia is an attempt to find the largest living creature on Earth."
Lynn Margulis, the coauthor of Gaia hypotheses, is more careful to avoid controversial figures of speech than is Lovelock. In 1979 she wrote, in particular, that only homeorhetic and not homeostatic balances are involved: that is, the composition of Earth's atmosphere, hydrosphere, and lithosphere are regulated around "set points" as in homeostasis, but those set points change with time. Also she wrote that there is no special tendency of biospheres to preserve their current inhabitants, and certainly not to make them comfortable. Accordingly, the Earth is not a living organism which can live or die all at once, but rather a kind of community of trust which can exist at many discrete levels of integration. But this is true of all multicellular organisms, not all cells in the body die instantaneously.
 Critical analysis
This theory is based on the simple idea that the biomass self-regulates the conditions on the planet to make its physical environment (in particular temperature and chemistry of the atmosphere) on the planet more hospitable to the species which constitute its "life". The Gaia Hypothesis proper defined this "hospitality" as a full homeostasis. A simple model that is often used to illustrate the original Gaia Hypothesis is the so-called Daisyworld simulation.
Recent scientific testing of homeostatic feedbacks involving plankton in Arctic and Antarctic waters, and the effects of rainforests in cloud formation suggest effective regulation of planetary albedo is being compromised by global warming and rainforest destruction.
Whether this sort of system is present on Earth is still open to debate. Some relatively simple homeostatic mechanisms are generally accepted. For example, when atmospheric carbon dioxide levels rise, plants are able to grow better and thus remove more carbon dioxide from the atmosphere, but the extent to which these mechanisms stabilize and modify the Earth's overall climate are not yet known. Less clear is the reason why such traits evolve in a system in order to produce such effects. Lovelock accepts a process of systemic Darwinian evolution for such mechanisms, creatures that evolve that improve their environment for their survival will do better than those which damage their environment. But many Darwinists have difficulty accepting such mechanisms can exist.
After initially being largely ignored by scientists, (from 1969 till 1977), thereafter for a period, the initial Gaia hypothesis was ridiculed by some scientists. On the basis of its name alone, the Gaia hypothesis was derided as some kind of neo-Pagan New Age religion. Many scientists in particular also criticised the approach taken in his popular book "Gaia, a New look at Life on Earth" for being teleological; a belief that all things have a predetermined purpose. Lovelock seems to have accepted this criticism of some of his statements, and has worked hard to remove the taint of teleological purpose from his theories, stating "Nowhere in our writings do we express the idea that planetary self-regulation is purposeful, or involves foresight or planning by the biota." – (Lovelock, J. E. 1990).
In 1981, W. Ford Doolittle, in the CoEvolution Quarterly article "Is Nature Motherly" argued that there was nothing in the genome of individual organisms which could provide the feedback mechanisms Gaia theory proposed, and that therefore the Gaia hypothesis was an unscientific theory of a maternal type without any explanatory mechanism. In 1982 Richard Dawkins in his book The Blind Watchmaker argued that organisms could not act in concert as this would require foresight and planning from them. Like Doolittle he rejected the possibility that feedback loops could stabilize the system. Dawkins claimed "there was no way for evolution by natural selection to lead to altruism on a Global scale".
Stephen Jay Gould criticised Gaia as merely a metaphorical description of Earth processes. He wanted to know the actual mechanism by which self-regulating homeostasis was regulated. David Abram was to rebut Gould's claim by arguing that reductionistic science is itself based upon the metaphore of a clockwork machine. Darwinian evolution itself is based upon a "natural selection" analogous to the artificial selection of plant and animal breeders. Others have shown that as our machines become more cybernetic and microbiology discovers organic feedback systems the old organic-mechanical metaphorical split becomes less meaningful.
A final criticism leveled against the idea that Gaia is a "living" organism is the fact that the planet has not and is unable to reproduce. Certainly one of the hallmarks of living organisms is their ability to replicate and pass on their genetic information to succeeding generations. But the same criticism could be levelled at a yeast culture. We have no difficulty recognising the fact that they are alive even though they do not sexually reproduce. Other Gaians have proposed that Gaia is still too young to reproduce and this is not to say that it is conceptually impossible, as humankind may be the means by which Gaia will reproduce. Humanity's exploration of space, its interest in colonizing other planets, and the large body of sci-fi literature that describes terraforming, lend strong evidence to the idea that Gaia is planning to reproduce. The astronomer Carl Sagan also remarked that from a cosmic viewpoint, the space probes since 1959 have the character of a planet preparing to go to seed.