"Surface conditions on Earth, have been for most of geological time regulated by life……This new link between Geology and Biology originated in the Gaia hypothesis''
NASA geologist Paul Lowman (2002)
In 1965 James Lovelock, inspired by research on the habitability of Mars, proposed in a Nature-article to consider the various spheres of earth (lithosphere, hydrosphere, biosphere and atmosphere) as an interconnected, self-regulating system. He followed the suggestions by novelist William Golding and named this idea the controversial Gaia-hypothesis, after the ancient titan Gaia - the personification of earth.
Lovelock argued that biotic and abiotic processes limit the possible amplitude of changes of terrestrial conditions - like the salinity of the oceans, the surface temperature and the atmospheric chemistry - therefore forcing earth into a life-supporting disequilibrium between two stable extremes, the frozen wasteland of Mars and a greenhouse hell of Venus.
"Some 30 million types of extant organisms have descended with modification from common ancestors; that is, all have evolved. All of them-ultimately bacteria or products of symbioses of bacteria - produce reactive gases to and remove them from the atmosphere, the soil, and the fresh and saline waters. All directly or indirectly interact with each other and with the chemical constituents of their environment, including organic compounds, metal ions, salts, gases, and water. Taken together, the flora, fauna, and the microbiota (microbial biomass), confined to the lower troposphere and the upper lithosphere, is called the biota. The metabolism, growth, and multiple interactions of the biota modulate the temperature, acidity-alkalinity, and, with respect to chemically reactive gases, atmospheric composition at the Earth's surface."
However the general notion that the Gaia-hypothesis states that "earth as a living planet" or a "life form" in the sense of entity is incorrect.
Organism do not manipulate deliberately the system so it can support them; however if an organisms harms his environment (and the life-supporting properties) it eventually will be naturally selected from the system. Fortunately environments can also tolerate some degree of change without losing their life-supporting properties.
The legacy of the Gaia-hypothesis is the consideration to see geology as one of the Earth System Sciences and appropriately to consider what we call "Earth" as the result of "elegant truths; of exquisite interrelationships; of the awesome machinery of nature." (Carl Sagan in episode 1 of Cosmos "The Shores of the Cosmic Ocean", 1990)
MARGULIS, L. (2004): Gaia by Any Other Name. In (ed.), Schneider S.H.: "Scientists Debate Gaia - The Next Century": 7 - 12