Theory that Earth is not a dead mass of rock but instead a living organism. The Gaia hypothesis was first proposed by Timothy Zell in 1970 and refined by James Lovelock and Lynn Margulis in 1972. According to Lovelock, Earth escaped the fate of Venus and Mars because it was occupied about 3 billion years ago by a life form that began to transform the planet itself into its own matter. All life forms that have evolved on Earth, according to the Gaia hypothesis, are part of the Gaia organism, just as cells make up a human body. Like these cells, the different forms of life that make up Gaia interact to contribute to the health of the whole organism.
Lovelock developed the Gaia hypothesis from information gathered by the unmanned Viking probes sent by NASA to Mars in the 1970s. He theorized, based on the atmospheric composition of Mars, that life could not exist on the planet. The Viking landers confirmed that life as we know it does not exist on the Martian surface, although scientists have come to believe that at one time in the distant past simple forms of life probably did live on the planet. They reject Lovelock’s thesis on this basis. Lovelock applied his ideas to the Earth, reasoning that, had a Viking lander arrived in Antarctica, it would have found few signs of life there as well. Looking at the world as an alien in search of signs of life would do, Lovelock concluded that the Earth itself was not so much a planet inhabited by living species as a holistic self-sustained and self-regulating system. In other words, the Earth was itself a living organ¬ ism. Lovelock named this organism Gaia, after the earth-mother who gave birth to the Titans in Greek myth.