Imagine a science-based civilization far distant in the Galaxy that had built an interferometer of such resolving power that it could analyse the chemical composition of our atmosphere. Simply from this analysis, they could confidently conclude that Earth, alone among the planets of the Solar System, had a carbon-based life and an industrial civilization. They would have seen methane and oxygen coexisting in the upper atmosphere, and their chemists would have known that these gases are continually consumed and replaced. The odds of this happening by chance inorganic chemistry are very long indeed. Such persistent deep atmospheric disequilibrium reveals the low entropy characteristic of life. They would conclude that ours was a live planet — and the presence of CFCs in the atmosphere would suggest an industry unwise enough to have allowed their escape.
As part of NASA's planetary exploration team in 1965, thoughts such as these led me to propose atmospheric analysis for detecting life on Mars. I also wondered what could be keeping Earth's chemically unstable atmosphere constant and so appropriate for life, and what kept the climate tolerable despite a 30% increase in solar luminosity since the Earth formed. Together, these thoughts led me to the hypothesis that living organisms regulate the atmosphere in their own interest, and the novelist William Golding suggested Gaia as its name. Although the concept of a live Earth is ancient, Newton was the first scientist to compare the Earth to an animal or a vegetable. Hutton, Huxley and Vernadsky expressed similar views but, lacking quantitative evidence, these earlier ideas remained anecdotal. In 1925 Alfred Lotka conjectured that it would be easier to model the evolution of organisms and their material environment coupled as a single entity than either of them separately. Gaia had its origins in these earlier thoughts, from the evidence gathered by the biogeochemists Alfred Redfield and Evelyn Hutchinson and from the mind-wrenching top-down view provided by NASA.
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