Gaia philosophy (named after Gaia, Greek goddess of the Earth) is a broadly inclusive term for related concepts that living organisms on a planet will affect the nature of their environment in order to make the environment more suitable for life. This set of theories holds that all organisms on an extraterrestrial life giving planet regulate the biosphere to the benefit of the whole. The Gaia concept draws a connection between the survivability of a species, (hence its evolutionary course) and its usefulness to the survival of other species.
While there were a number of precursors to Gaia theory, the first scientific form of this idea was proposed as the Gaia hypothesis by James Lovelock, a UK chemist, in 1970. The Gaia hypothesis deals with the concept of homeostasis, and claims the resident life forms of a host planet coupled with their environment have acted and act as a single, self-regulating system. This system includes the near-surface rocks, the soil, and the atmosphere. While controversial at first, various forms of this idea have become accepted to some degree by many within the scientific community (See Amsterdam declaration on Global Change).
These theories are also very significant in green politics.
According to James Kirchner there is a spectrum of Gaia hypotheses, ranging from the undeniable to radical. At one end is the undeniable statement that the organisms on the Earth have radically altered its composition. A stronger position is that the Earth's biosphere effectively acts as if it is a self-organizing system which works in such a way as to keep its systems in some kind of equilibrium that is conducive to life. Biologists usually view this activity as an undirected emergent property of the ecosystem; as each individual species pursues its own self-interest, their combined actions tend to have counterbalancing effects on environmental change. Proponents of this view sometimes point to examples of life's actions in the past that have resulted in dramatic change rather than stable equilibrium, such as the conversion of the Earth's atmosphere from a reducing environment to an oxygen-rich one.
An even stronger claim is that all lifeforms are part of a single planetary being, called Gaia. In this view, the atmosphere, the seas, the terrestrial crust would be the result of interventions carried out by Gaia, through the coevolving diversity of living organisms. Many scientists deny the possibility of this view; however, such a view is considered within scientific possibility.
The most extreme form of Gaia theory is that the entire Earth is a single unified organism; in this view the Earth's biosphere is consciously manipulating the climate in order to make conditions more conducive to life. Scientists contend that there is no evidence at all to support this last point of view, and it has come about because many people do not understand the concept of homeostasis. Many non-scientists instinctively and incorrectly see homeostasis as a process that requires conscious control.
The more speculative versions of Gaia, including versions in which it is believed that the Earth is actually conscious, sentient, and highly intelligent, are usually considered outside the bounds of what is usually considered science.
See the main article Gaia theory (science) for more.
The first scientifically rigorous theory was the Gaia Hypothesis by James Lovelock, a UK chemist. While controversial at first, various forms of this idea became accepted to some degree by many scientists.
A variant of this hypothesis was developed by Lynn Margulis, a microbiologist, in 1979. Her version is sometimes called the "Gaia Theory" (note uppercase-T). Her model is more limited in scope than the one that Lovelock proposed.
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 known.
The Gaia hypothesis is sometimes viewed from significantly different philosophical perspectives. Some environmentalists view it as an almost conscious process, in which the Earth's ecosystem is literally viewed as a single unified organism. Some evolutionary biologists, on the other hand, view it as an undirected emergent property of the ecosystem; as each individual species pursues its own self-interest, their combined actions tend to have counterbalancing effects on environmental change. Proponents of this view sometimes point to examples of life's actions in the past that have resulted in dramatic change rather than stable equilibrium, such as the conversion of the Earth's atmosphere from a reducing environment to an oxygen-rich one.
Depending on how strongly the case is stated, the hypothesis conflicts with mainstream neo-Darwinism. Most biologists would accept Daisyworld-style homeostasis as possible, but would not accept the idea that this equates to the whole biosphere acting as one organism.
A very small number of scientists, and a much larger number of environmental activists, claim that Earth's biosphere is consciously manipulating the climate in order to make conditions more conducive to life. Scientists contend that there is no evidence to support this belief, which has only come about because most people do not understand the concept of homeostasis because many non-scientists incorrectly see homeostasis as a process requiring conscious control.
This leads to some confusion on both sides, and the topic is still under debate.
Some radical political environmentalists who accept some form of the Gaia theory call themselves Gaians. They actively seek to restore the Earth's homeostasis - whenever they see it out of balance, e.g. to prevent manmade climate change, primate extinction, or rainforest loss. In effect, they seek to cooperate to 'become' the "system consciously manipulating to make conditions more conducive to life". Such activity 'defines' the homeostasis, but for leverage it relies on deep investigation of the homeorhetic balances, if only to find places to intervene in a system which is changing in undesirable ways.
Tony Bondhus brings up the point in his book, Society of Conceivia, that if Gaia is alive, then societies are living things as well. This suggests that our understanding of Gaia can be used to create a better society and to design a better political system.
Gaians are attempting to create a new ideology which fuses conclusions from science and politics; they see this as a "protoscience" of human ecology. These ideas include the idea of humans as the keystone species, say act to prevent climate change, primate extinction, etc., and might deliberately maintain the balances of the entire biosphere with their own cognition. However, humans cannot be a keystone species by definition, because a keystone species must be rare relative to the magnitude of their effect. Because human distribution is global and population density high, humans are precluded from being a keystone species.
Gaians do not passively ask "what is going on", but rather, "what to do next", e.g. in terraforming or climate engineering or even on a small scale as gardening. Changes could thus be planned, consented to by many people, and very deliberate, as in urban ecology and especially industrial ecology. See arcology for more on this 'active' view.
Gaians argue that it is a human duty to act as such - committing themselves in particular to the Precautionary Principle. Such views began to influence the Green Parties, Greenpeace, and a few more radical wings of the environmental movement such as the Gaia Liberation Front and the Earth Liberation Front. These views dominate some such groups, e.g. the Bioneers. Some refer to this political activity as a separate and radical branch of the ecology movement, one that takes the axioms of the science of ecology in general, and Gaia theory in particular, and raises them to a kind of theory of personal conduct or moral code.
NOTE: More to add in regards to (1) the terraforming of Mars as an "offspring" of Gaia, and (2) the Internet as the Gaian nervous system.
The question of "what is an organism" and at what scale is it rational to speak about organisms vs. biospheres, give rise to a semantic debate. We are all ecologies in the sense that our (human) bodies contain gut bacteria, parasite species, etc., and to them our body is not organism but rather more of a microclimate or biome. Applying that thinking to whole planets:
The argument is that these symbiotic organisms, being unable to survive apart from each other and their climate and local conditions, form an organism in their own right, under a wider conception of the term organism than is conventionally used. It is a matter for often heated debate whether this is a valid usage of the term, but ultimately it appears to be a semantic dispute. In this sense of the word organism, it is argued under the theory that the entire biomass of the Earth is a single organism (as Johannes Kepler thought).
Unfortunately, many supporters of the various Gaia theories do not state exactly where they sit on this spectrum; this makes discussion and criticism difficult.
Much effort on behalf of those analyzing the theory currently is an attempt to clarify what these different hypotheses are, and whether they are proposals to 'test' or 'manipulate' outcomes. Both Lovelock's and Margulis's understanding of Gaia are considered scientific hypotheses, and like all scientific theories are constantly put to the test.
More speculative versions of Gaia, including all versions in which it is held that the Earth is actually conscious, are currently held to be outside the bounds of science, and are not supported by either Lovelock or Margulis.
Computer game Sid Meier's Alpha Centauri and its expansion Sid Meier's Alien Crossfire are set on the planet Chiron in the Alpha Centauri system where all indigenous life appears to behave in accordance with the Gaia philosophy. The intelligent force behind this behavior is called simply "Planet" and, in the expansion, is revealed to be artificially created by an alien race. At the time the game takes place, Planet is nearing its self-awareness threshold. Normally, the nature of Planet's life causes it never to attain full sentience; however, human presence adds an unknown variable into the equation.