The Gaea hypothesis is the theory, first advanced by James Lovelock in 1969, that the entire mass of living matter on Earth (or any planet with life) functions as a vast organism (which he named for the Greek goddess Gaia) that actively modifies its planet to produce the environment that suits its needs. This is similar to the Unification Church teaching that the creation is one interconnected body of beings with dual purposes [1].
It was later developed into the Gaia Theory of Lynn Margulis.
A simple model that is often used to illustrate the original Gaea hypothesis is the so-called "Daisyworld" simulation. A simulated planet is seeded with two different species of daisy as its only life form; one species has white flowers which reflect light very well and grows best in warm temperatures, and the other species has black flowers that absorb light well and grows best in cool temperatures. Whenever the planet's temperature decreases, the black flower predominates and raises temperatures back towards an equilibrium, and whenever the planet's temperature increases the white flower predominates and lowers the temperature again. Such a system is remarkably stable against varying solar input; the entire planet maintains homeostasis.
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 Gaea 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.