Dorion Sagan: A Body in the Form of a Planet

Gaia is a body in the form of a planet. Gaia describes a living Earth, an idea with precedents in natural science and philosophy for 2,500 years, and longer in many indigenous belief systems. The basis of the Gaia hypothesis is that Earth’s atmosphere is as complicated as the blood or skin of an animal. It was formally proposed by chemist James E. Lovelock (1919–2022) in 1972, and its name was provided by his neighbour, William Golding, author of Lord of the Flies. Lovelock conjectured that Earth’s surprisingly metastable chemistry was maintained by life’s activity. Other environmental variables, such as global mean temperature and marine salinity, and even the continuing presence of water on the planet, are theorised to be partly dependent on the activity of live organisms, effectively making the entire planetary surface alive. The evolutionary biologist Lynn Margulis (1938–2011), my mother and long-time writing partner, helped turn Lovelock’s hypothesis into a full-fledged theory: she posited that specifically gas-exchanging microbial life, which has been present on Earth’s surface for billions of years, is responsible for giving our planet the character of a living body.

Margulis’s former husband, Carl Sagan (1934–2006), the astrophysicist, astrobiologist and emerging telegenic spokesperson for post-Space Age science and humanism, first introduced Lovelock’s work to Margulis in a letter dated 16 June 1970, sent from Cornell. By then the two had broken up, but they were still in correspondence (my father addressed her as Dr Lynn Sagan after they split but then Mrs Lynn Margulis after she remarried). She had asked him whether he knew anyone working on the composition of Earth’s early atmosphere. He brought Margulis and Lovelock into contact with each other, serving as a scientific matchmaker for the development of Gaia theory, which benefitted from the blend of Margulisian microbial biology and Lovelockian atmospheric chemistry, although he remained sceptical of Lovelock’s thesis that Earth was a self-regulating planet. It is perhaps ironic that the person to broker the meeting was the most visible and vigorous public spokesperson for the search for life beyond Earth in the late twentieth century.*

In 1984, Margulis and I published a co-written article called ‘Gaia and Philosophy’.[1] It made a link between the idea of Gaia, whose name is mythological but whose impulse is scientific, and philosophy. Looking back on this essay some forty years on, after the death of its second author, is an opportunity to take stock. Gaia can be considered the greatest success of technological humankind’s attempts to find life elsewhere in space. It is the discovery that any living planet, insofar as it is like Earth, would be a self-regulating system. And, further, that an actively regulated planetary atmosphere that is not in thermodynamic equilibrium is a strong indication that life may be present at the surface. The contrast with traditional Western geological and geochemical ideas could not be more stark: Earth is no more a rock with some life on it than you are a skeleton infested with cells.

Lovelock and Sagan shared an office at the Jet Propulsion Laboratory in Pasadena, California in the late 1960s, where Lovelock had been hired by NASA to study the detection of life on Mars. Viking’s spectroscopic results of Mars’s atmosphere showed that it was mostly carbon dioxide, with only trace amounts of oxygen. This was in striking contrast to Earth, whose atmosphere is very complex and contains many compounds that should not even exist given the ordinary rules of chemical mixing. Comparing the atmospheric compositions of Mars and Earth, Lovelock felt sure no life was on Mars. Joshua Lederberg, one of the lead scientists working at the laboratory in the lead-up to the Viking Mars missions – the goal of which was to investigate life on Mars – coined the term ‘exobiology’ to describe the scientific search for life beyond Earth. In a letter from the early 1970s, Lovelock told Margulis that what the two of them were doing was not really exo- but esobiology: studies of planetary life focused not on space but on home. The blue planet was so rich in prokaryotes, bacteria and archaea, protists and fungi and photosynthesizers, that it was as worthy of study as outer space.[2]

Lovelock was an inventor of instruments that could detect trace gases, including an electron capture detector that allowed the measurement of human-produced hydrocarbons, such as DDT and PCB toxins, in pesticides and consumer products. Thus the reasoning by which he arrived at the Gaia hypothesis was thermodynamic, considering what accounted for the massive chemical and thermodynamic disequilibrium of Earth’s atmosphere. He reasoned that Earth’s atmosphere contains gases, both exotic and simple, which react with oxygen. For example, methane reacts with oxygen to form carbon dioxide and water. Yet methane and oxygen remain in the atmosphere. What keeps them there? Living beings, Lovelock surmised, used the atmosphere as a sort of external circulatory system, and their activities were observable at a planetary level, as they created measurable quantities of reactive gases in Earth’s atmosphere. If life existed elsewhere, organisms would do the same in alien
biospheres. The Martian atmosphere, over 95 per cent carbon dioxide, was in chemical equilibrium; its current atmosphere was the end-product of chemical reactions, like the ashes of a fire. There was no need to go there, he told his life-seeking colleagues and NASA bosses, mischievously saying they could save a lot of money: Mars never had life, or if it did, was now a dead planet.

The Gaian commentator Bruno Latour compared Lovelock’s electron capture device to a telescope pointed at Earth, showing Earth’s surface to belong to an organised, living thing. But Latour, with a Judeo-Christian tinge, further suggested that the discovery of Gaia represents a reversal of the Galilean and Copernican notion of the open universe of many planets, toward a new perspective that recognises Earth as special. However, exactly the opposite is the case. Earth’s life, and life in general, is a form of nonequilibrium system which, like other such nonliving complex systems, reduces ambient gradients (differences across a distance), cycles matter internally and maintains form, while producing entropy (a measure of the spread of energy) in exemplary conformity with thermodynamics’ second law. Although life on Earth, or rather, the life of Earth, gives its character to the entire planet – making it distinct from other planets so far examined by astronomers – the living process it exhibits, far from being unique, is that of all open thermodynamic systems that feed on available energy and produce wastes – ultimately heat. Due to the aggregate sensing and complex chemical activities of its inhabitants, Earth’s complex thermodynamic system[3] exhibits natural purpose[4] – even a kind of mind.

Excerpted from the Introduction to Gaia and Philosophy.


1 Sagan, D. and L. Margulis. 1984. ‘Gaia and Philosophy’ in L. Rouner (ed) On Nature
(University of Notre Dame Press)
2 Clarke, B. and S. Dutreuil. 2022. Writing Gaia: The Scientific Correspondence of James Lovelock
and Lynn Margulis (Cambridge University Press)
3 Schneider, E. D. and D. Sagan. 2005. Into the Cool: Energy Flow, Thermodynamics, and Life
(University of Chicago Press)
4 Sagan, D. and J. H. Whiteside. 2008. ‘Gradient-Reduction Theory: Thermodynamics and
the Purpose of Life’ in Schneider, S., J. Miller, E. Crist and P. Boston (eds) Scientists Debate
Gaia: The Next Century (MIT Press)