The Gaia hypothesis was first suggested in 1969 by the British biologist James Lovelock (1919- )—with the assistance of the biologist Lynn Margulis (1938- ). This interesting hypothesis states that Earth's biosphere has an important modulating effect on the terrestrial atmosphere. Because of the chemical complexity observed in the lower atmosphere, Lovelock postulated that life-forms within the terrestrial biosphere actually help control the chemical composition of the Earth's atmosphere, thereby ensuring the continuation of conditions suitable for life. Gas-exchanging microorganisms, for example, are thought to play a key role in this continuous process of environmental regulation. Without these "cooperative" interactions in which some organisms generate certain gases and carbon compounds that are subsequently removed and used by other organisms, planet Earth might also possess an excessively hot or cold planetary surface that is devoid of liquid water and is surrounded by an inanimate, carbon dioxide-rich atmosphere.
Gaia (also spelled Gaea) was the goddess of Earth in ancient Greek mythology. Lovelock used her name to represent the terrestrial bio-sphere—namely, the system of life on Earth, including living organisms and their required liquids, gases, and solids. Thus, the Gaia hypothesis metaphorically implies that "Gaia" (Earth's biosphere) will struggle to maintain the atmospheric conditions that are suitable for the survival of terrestrial life.
If scientists use the Gaia hypothesis in their search for extraterrestrial life, they should look for extrasolar planets that exhibit variability in atmospheric composition. Extending this hypothesis beyond the terrestrial biosphere, a planet will either be living or else it will not. The absence of chemical interactions in the lower atmosphere of an alien world could be taken as an indication of the absence of living organisms.
Although this interesting hypothesis is currently more speculation than hard, scientifically verifiable fact, it is still quite useful in developing a sense of appreciation for the complex chemical interactions that have helped to sustain life in the Earth's biosphere. These interactions among microorganisms, higher-level animals, and their mutually shared atmosphere might also have to be carefully considered in the successful development of effective closed life-support systems for use on permanent space stations, lunar bases, and planetary settlements.
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