The concept of Gaia, a self-regulating Earth, excites both admiration and obloquy. Its inventor (or rather rediscoverer) describes the genesis and evolution of the hypothesis.

In essence, the Gaia hypothesis was born 25 years ago. Those who have since worked closely with it see it as reasonable science and growing ever more plausible as evidence and models come together. But theirs is a minority opinion. Most scientists regard Gaia as little more than metaphor; some even denounce it as antiscience. This Commentary is about the history, the arguments, the evidence and the criticisms of Gaia, and about where it now stands.

Beginnings

The history of the idea goes back to the mid-1960s, to the Jet Propulsion Laboratory in California. The National Aeronautics and Space Administration had sought my help in the quest to discover whether there was life on Mars, and in 1965 I proposed some physical tests for the presence of planetary life'. One of these was a top-down view of the whole planet instead of a local search at the site of landing. The test was simply to analyse the chemical composition of the planet's atmosphere. If the planet were lifeless, then it would be expected to have an atmosphere determined by physics and chemistry alone, and be close to the chemical equilibrium state. But if the planet bore life, organisms at the surface would be obliged to use the atmosphere as a source of raw materials and as a depository for wastes. Such a use of the atmosphere would change its chemical composition. It would depart from equilibrium in a way that would show the presence of life.

At that time Dian Hitchcock joined me and together we examined atmospheric evidence from the infrared astronomy of Mars. We compared this evidence with that available about the sources and sinks of the gases in the atmosphere of the one planet we knew bore life, Earth. We found an astonishing difference. The atmosphere of Mars was close to chemical equilibrium and dominated by carbon dioxide, but that of Earth was in a state of deep chemical disequilibrium. In our atmosphere carbon dioxide is a mere trace gas. The coexistence of abundant oxygen with methane and other reactive gases would be impossible on a lifeless planet. Even Earth's abundant nitrogen and water are difficult to explain by geochemistry. There are no such anomalies in the martian and venusian atmospheres and their existence in the Earth's atmosphere signals the presence of living organisms at the surface. Sadly, we concluded, Mars is probably lifeless.

That first sight from space of a dappled white-and-blue sphere irreversibly altered most people's emotions about the Earth. Atmospheric chemistry, seen from above, was in scientific terms for me a similar revelation about the Earth. This top-down analysis revealed the atmosphere as a gas mixture like that of the intake manifold of an internal combustion engine, with oxygen and combustible gases mixed, and very different from the exhausted carbon-dioxide-dominated atmospheres of Mars and Venus. Further, I knew that the chemical composition of the atmosphere was stable for long periods when compared with the residence times of its gases. One afternoon in 1965, thinking about these facts, the thought came that such constancy requires the existence of an active control system.

At that time I lacked any idea of the nature of this control system, except that the organisms on the Earth's surface were part of it. I learned from astrophysicists that stars increase their heat output as they age and that our Sun has grown in luminosity by 25 per cent since life began. I realized that, in the long term, climate also might be actively regulated. The notion of a control system involving the whole planet and the life upon it was now firmly established in my mind. Sometime near the end of the 1960s I discussed this idea with my near neighbour, the novelist William Golding. He suggested the name Gaia as the only one appropriate for so powerful an entity. Here I shall use the word Gaia as shorthand for the theory.

I first formally put forward the idea of Gaia as a control system in 1972. Shortly afterwards I began to work with Lynn Margulis, a collaboration that has continued. We first stated our hypothesis in words such as "Life, or the biosphere, regulates or maintains the climate and the atmospheric composition at an optimum for itself". By 1973 we refined the hypothesis and re-stated Gaia in papers in Tellus and Icarus as "The notion of the biosphere as an adaptive control system that can maintain the Earth in homeostasis, we are calling the Gaia hypothesis".

Teleology

Neither Lynn Margulis nor I have ever proposed a teleological hypothesis. Nowhere in our writings do we express the idea that planetary self-regulation is purposeful, or involves foresight or planning by the biota. It is true that our early statements about Gaia were imprecise and open to misinterpretation, but this does not justify the persistent almost dogmatic, criticism that our hypothesis is teleological. Sometimes the criticism reveals an ignorance of the circular logic of control theory&emdash;how often are the terms negative and positive feedback used, through lack of understanding, as mere metaphors?

As an inventor I know it is easy to image, construct and then reduce a selfregulating device to practice. I also know how difficult it is to explain it in words or even in analytical mathematics. In many ways Gaia, like an invention, is difficult to describe. The nearest I can reach is to call Gaia the theory of an evolving system&emdash;a system made from the living organisms of the Earth, and from their material environment, the two parts being tightly coupled and indivisible. This evolutionary theory views the self-regulation of climate and chemical composition as emergent properties of the system. The emergence is entirely automatic; no teleology is invoked. Gaia evolves as a system, gradually, during long periods of homeostasis that are punctuated by sudden simultaneous changes in both organisms and environment. Such changes move the system to new and different homeostatic states; a significant jump of this kind occurred between the anaerobic Archaean and the oxygenated Proterozoic 2.5 thousand million years ago. Gaia offers a resolution of the long debate over whether evolution was gradual or punctuated by suggesting that it was both organisms

This is not a new theory of evolution. James Hutton intuited it when, in 1788, he saw the Earth as a super-organism which could only be studied properly by physiology. Alfred Lotka also expressed it in 1925 when he clearly stated that the evolution of organisms could not be separated from the evolution of their physical environment. Gaia theory is not incompatible with Darwin's great vision; it includes the evolution of the organisms by natural selection as an essential part of a self-regulating planet.

Much of the confusion about Gaia and darwinism comes from the misuse of the concept of adaptation. My friend and colleague Andrew Watson succinctly expressed the step that distinguishes Gaia from darwinism in a debate before the Linnean Society in December 1989. It lies in the tightness of the coupling between the organisms and their physical environment. Watson observed that almost everyone now accepts that life profoundly influences the environment; this is now the conventional wisdom among geochemists, and a considerable change from their view pre-Gaia. It is equally obvious that life is influenced by and adapts to the environment. This is the older wisdom that has prevailed throughout this century. Therefore, as Watson pointed out, life and the environment are a coupled feedback system, where changes in one element will affect the other and this may in turn feed back on the original change. The real debate, then, is how important and how tight is the coupling? Does it, as we believe, confer new properties on the system, such as enhanced stability or behaviour similar to that of a living organism?

Holism

Gaia asserts that this close coupling of organisms and their environment is strong enough to have greatly influenced the way in which the life-environment system on Earth, and on other planets with life, has evolved. It is strong enough that we will not properly understand Earth history until we think of the system as just that, a whole system, and stop trying to understand its parts in isolation from one another.

Like living organisms generally, Gaia is an open but bounded system. On one side the bound is space and Gaia exchanges radiation across it. The other bound is the base of the crustal rocks where matter exchanges with the near-infinite mass of Earth's mantle. Gaia's wastes are low quantum-energy infrared radiation that escapes to space, and rock subducting to merge with the mantle. The profound disequilibrium of the atmosphere is one measure of the entropy reduction of this system. I think that James Hutton was right to call the Earth a super-organism and to regard physiology as the proper science for its investigation.

In 1982, Richard Dawkins made the strong criticism that no way exists for evolution by natural selection to lead to altruism on a global scale. My answer was the numerical model, daisyworld. The model is of a simple planet that keeps its climate constant in the face of ever increasing solar output. It does so by the competition for space of two species of daisies, one dark and one light. The planetary albedo is tightly coupled to the evolution of the daisies, and the evolution of the daisies is tightly coupled to the evolution of the climate.

More sophisticated models have been developed that go beyond daisyworld and illustrate the simultaneous self-regulation of climate and chemistry by bacterial ecosystems. These new models of Gaia are general and not limited as are models drawn from the separated disciplines of science. They can include many species or ecosystems simultaneously and follow their regulation as well as that of the material environment. Population biologists, for 60 years, have been unable to model more than two species simultaneously. The box models of biogeochemists are also unstable and unduly sensitive to the choice of initial conditions. These modellers handicap themselves by failing to include the biota, or the material environment, interactively.

Some geochemists have been opponents of Gaia from the beginning. Their criticism is the reasonable one that Gaia is not needed to explain the facts of the Earth. Yet these scientists are ready to tolerate Gaia as a theory under test and interesting as a source of ideas for new experiments. In an act of courage and generosity, the climatologist Stephen Schneider organized a conference of the American Geophysical Union in 1988 solely to discuss Gaia. The conference is described in Schneider's book Global Warming. This meeting marked the end of the false accusation of teleology&emdash;what some, illogically, have called the strong Gaia hypothesis. At the conference, J.W. Kirchner made a spirited attempt to demolish all notions of Gaia. Like some figure of the Inquisition, he publicly burned several imaginary Gaias, and his pyrotechnic demolition of the strong Gaia stole the show. But when the sparks faded, the real system Gaia was still there hidden only by the smoke. The flux of papers inspired by Gaia, and now appearing in the journals, are the real proof of the value of the conference. It has not stopped peer review from censoring any mention of Gaia by name.

The value of Gaia as a theory is illustrated by the interpretation of the long-term history of climate it makes possible. The conventional wisdom about the problem of the cool early Sun is that a warmth sufficient for the start of life, some 3.6-4 thousand million years ago, came from a greenhouse blanket of atmospheric CO2. This accounts for the start, but there has been no satisfactory explanation of the constancy of the Earth's climate throughout history.

Geophysiology

Geophysiology&emdash;my own neologism, but a useful one I think&emdash;can provide such an explanation. In a model of the Archaean period, a symbiotic pair of ecosystems, photosynthesizers and anaerobes, evolve tightly coupled to their physical and chemical environment. The model system sustains a climate and atmospheric composition comfortable for Archaean life. The photosynthesizers tend to cool by removing CO2, both directly and by increasing the rate of weathering. The anaerobes tend to warm by producing the greenhouse gas methane. By including a third ecosystem (consumers), the Archaean model is extended into the Proterozoic without loss of self regulation. These models are stable, indifferent to the choice of initial conditions and can resist perturbation.

Among the insights that come from a gaian approach are that planetary life can never be sparse. A planet with sparse life could never self regulate. The geophysical and geochemical evolution of the terrestrial planets is progressive and towards states like those of Mars and Venus now. During this evolution there will be a period when conditions are favourable for life and, to survive, the organisms must become abundant enough to affect and become coupled to the geochemical evolution; mere adaptation is not enough. If they fail, planetary conditions will continue to change inorganically until the point is reached when life is impossible.

Another insight from Gaia is that the biota are continuously engaged in the removal of CO2 from the air and that the present cool climate results from such biological pumping; viewed in this way, the warmth of the present interglacial perhaps implies a less-abundant or less-efficient biota. Gaia theory also led directly to the identification of another possible climate control system&emdash;the global-scale emission of dimethyl sulphide from the oceans, which was discovered in the course of a search for a chemical agent of biological origin to complete the natural sulphur cycle. This process could turn out to be as great a climate regulator as the carbon dioxide and methane greenhouse.

From a Northern Hemispheric human view we see glaciations as a disaster, but from a planetary viewpoint they may be a preferred state, one where life is more vigorous. When seen in this light, the current interglacial appears as a planetary fever, a pathology. In this context, pollution with greenhouse gases and the widespread destruction of natural habitats are further insults to a weakened system. Broecker has warned that geophysical processes alone could lead to "surprises in the greenhouse". As an active control system, Gaia can speed and intensify such surprises. A systems response might at first mask the expected warming by the greenhouse, for example by an actively increased cloudiless. We might then become complacent, or argue that the greenhouse has been overstated, only to be surprised by catastrophic climate change when the system is overwhelmed.

Testability

Gaia theory is testable, and the tests carried out so far are listed in the table. Why then has Gaia been so unpopular among scientists? An obvious reason would be the natural inertia of science which means that large-scale theories are digested only slowly. It took 40 years for another Earth theory, plate tectonics, to be accepted. With Gaia there are, I think, additional reasons for its unpopularity.

Gaia is developing normally in the Earth sciences and in time will be either accepted or rejected on the evidence. Some biologists by contrast denounced the idea even before evidence was available. Postgate referred to Gaia as "silly and dangerous.... pseudoscientific myth making". He went on to blame Gaia for the disrepute now attending science.

Even my friends among biologists become anxious when I talk of Gaia as an organism. Yet in so doing I merely continue a tradition that goes back at least as far as Leonardo da Vinci and that was made scientific by James Hutton. Biologists rejected this tradition in the nineteenth century when their subject first became intensely reductionist. Biology has seemed to be imprisoned in a narrow, almost puritan, reductionism, a consequence perhaps of the long and continuing war with vitalists, animists and religious fundamentalists. In war, it is said, the first casualty is truth. I think that science, when it engages in adversarial encounters with crude dogma or non-science, loses objectivity and itself grows dogmatic. In their criticisms, biologists seem to see Gaia as another threat to the sanctity of their creature, neo-darwinism. No one denies the power of reduction with its apotheosis in the triumphs of molecular biology. But the reductionists in their dogmatic crusade have made holism a pejorative term, and now cybernetics, Gaia and systems analysis are categorized with 'new age' philosophy and organic food. No wonder the greens prefer Gaia to biology.

A great loss results from this rejection of a rigorous top-down approach to systems. Holism is complementary not adversarial to reduction. What engineer now would dare to dissect a working computer into its hardware components before testing it as a whole system? Without holistic thinking, physiology, engineering and Gaia would be completely disabled.

The most difficult and the dirtiest criticism of Gaia is that there is nothing new in the idea, that it has all been said before&emdash;difficult, because there is much truth in it; dirty, because, as William James said of the fate of any new idea, "First, it's absurd, then maybe, and last, we have known it all along". Most biologists and many geochemists are ignorant of the details of control theory, the domain of engineers and physiologists. Merely for them to say it was explained by the balance of nature long ago is not enough. If it were, then explanations of why the concentration of oxygen is at 21 per cent, or of why the climate has remained favourable for 3.6 thousand million years, would already be available. Before the advent of the Gaia hypothesis, such questions were rarely asked, and would have been as pointless as asking an anatomist or a biochemist how human temperature is regulated. Such questions about systems cannot be answered from the separated disciplines of biochemistry or biogeochemistry, nor from neo-darwinist biology. The answer comes from physiology or control theory.

What of the criticism that Gaia gives support to anti-science? True, Gaia as a name, or a sign, has extended far beyond my intentions. As the semiotician Myrdene Anderson put it several years ago, "Gaia is an empty sign with near infinite capacity for signification". I watch it filling fast, and mostly with rubbish, like an empty skip left on a London street. Surely, though, this is the fate of any new sign and has nothing to do with the quality of the science of Gaia. I have offered 'geophysiology' as an alternative name, but so far there have been few takers.

I am not dismayed by the wide general interest in Gaia; rather, I am moved by the interest shown by theologians and philosophers. Whether or not Gaia is an accurate description of the Earth, it forces a different view from that of conventional wisdom&emdash;a view that could be crucial to understanding the consequences of pollution and environmental disturbance.

Arthur Clarke's notable comment, "How inappropriate to call this planet Earth when clearly it is Ocean", illustrates the wisdom of a top-down view. Few have been privileged, as astronauts, to see the Earth in its splendour from above. But anyone can rise above dogma, scientific or religious, and look down to see and cherish a most seemly Earth.

Back...