A Goddess of the Earth?
The Debate over the Gaia Hypothesis

Updated 12/01/2003

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"The Gaia hypothesis says that the temperature, oxidation state, acidity, and certain aspects of the rocks and waters are kept constant, and that this homeostasis is maintained by active feedback processes operated automatically and unconsciously by the biota."
- James Lovelock, The Ages of Gaia

Suggested Readings:

  • Kirchner, J.W. 1991. The Gaia hypothesis: Are they testable? Are they useful? In Schneider, S. (ed.) Scientists on Gaia. Cambridge, MA: MIT Press.
  • Margulis, L and J. Lovelock. 1976. Is Mars a Spaceship, Too? Natural History, June/July pp. 86-90

In this lesson, we learn:

  • What is the Gaia Hypothesis, and how does it relate to the idea of global change?
  • According to the hypothesis, how are the living and non-living components of earth related?
  • What evidence exists in support of the hypothesis? How have critics countered Gaia's explanations of earth phenomenon?
  • What are the weaknesses of the hypothesis? What are its strengths?
  • What are some examples of Gaia-like feedbacks?
Jump to: [Introduction] [Origin of the Hypothesis] [Examples of Regulation] [Alternatives to Gaia] [Many Gain Hypotheses] [Summary]

1. Introduction - Gaia and Global Change?

It often seems obvious that life on earth lives at the mercy of powerful non-biological forces like volcanic eruptions, storms, climate change, and even the movement of continents. Over this semester you have learned how we believe matter and the universe came about, how the solar system was formed, and how life on earth emerged and diversified. Today much of the earth's surface is covered by a layer of life, and everywhere on earth the influence of living organisms has an effect. Recently there has emerged a controversial theory, called the Gaia Hypothesis. It is based on the idea that, over the long run of geological time, life may control the powerful physical forces for its own good.

"The Gaia hypothesis states that the lower atmosphere of the earth is an integral, regulated, and necessary part of life itself. For hundreds of millions of years, life has controlled the temperature, the chemical composition, the oxidizing ability, and the acidity of the earth's atmosphere"
(Margulis, L and J. Lovelock. 1976. "Is Mars a Spaceship, Too?" Natural History, June/July pp. 86-90)


2. The Hypothesis and its Originators

The originators of the hypothesis were James Lovelock and Lynn Margulis. Lovelock is a British independent scientist and inventor with a background in human physiology. Margulis was, in the 1970's, a microbiologist at Boston University. She is also the originator of the theory that the eukaryotic cell arose by endosymbiotic cell capture - this was a radical idea that has become widely accepted, thereby giving Margulis a high degree of credibility.

The essential idea of the Gaia Hypothesis is analogous to the thermostat in your home, or the thermostat in your brain. You set the thermostat in your home to 65 F in order to keep a comfortable living environment. When the temperature falls below this, the furnace is switched on. When the temperature in the house reaches the target, the furnace is switched off. Something more complicated, but with similar effect, goes on in our bodies. Everyone of us is a comfy 98.6 F now, and almost always. If our body temperature deviates very far from a narrow range, we die. The human body has a number of self-regulatory, or homeostatic, mechanisms.

The conditions for life as we know it to exist also require a relatively narrow range of circumstances. How does life modify the physical and chemical conditions of the environment?


3. Examples of Regulation of the Environment, According to Gaia


  1. Where does oxygen come from? Small amounts emanate from volcanic activity, but usually it is combined with other elements, e.g., as CO2 and H20. As you have learned, the earth's original atmosphere contained almost no oxygen, and the advent of photosynthesis some 2.5 billion years ago is responsible for the presence of abundant oxygen in the atmosphere (presently 21%). As explained in lecture, initially the liberated oxygen combined with oxidizable minerals such as iron, leaving a sedimentary record of red bands that tells us that a new atmospheric chemistry was being brought about by life. Other geological evidence suggests that oxygen levels on earth have been, within a factor of roughly two, at near-present values for the past billion years, during which complex multi-cellular life arose.

    If oxygen were to reach a value of 30% of atmospheric gas composition, fires would occur whenever a lightening bolt hit humid vegetation. The planet would be in serious danger of burning up. What has kept oxygen from building up to dangerous levels? Why has it gone from nearly zero to 21%, and then stopped? One possible answer is the biological production of methane by bacteria. A short-lived molecule, methane might combine with oxygen to produce CO2, thus stabilizing oxygen concentrations.

  2. We know that climate has changed a great deal in the past, producing episodes of glaciation, and could warm significantly in the future, due to atmospheric pollution. However, climate change could be much more extreme. At least for the past billion years it is unlikely that the earth's temperature was more than 15 degrees warmer or 5 degrees colder than it is today. Earlier temperatures are very uncertain. Calculations suggest that the sun emitted perhaps 25% less heat energy some 4 billion years ago, than it does today. Calculations also suggest that under this faint early sun, the earth should have been a frozen ball. However, life arose under these conditions, and there is geological evidence of flowing water from this time. It has been suggested that a kind of greenhouse warming was in effect at that time, involving such gases as methane, ammonia, and carbon dioxide, and that this is evidence of a kind of Gaian planetary temperature control mechanism.

    And why hasn't the planet overheated, since the sun has increased in luminosity over the past 4 billion years? Lovelock and Margulis argue that life solved this one, also. A warming earth stimulated greater plankton production, removing CO2 from the atmosphere. When the plankton died they sank to the ocean floor, forming sediments, and thus removed CO2 from the system. Moreover, a warmer planet has more rain, which means more erosion and more nutrient runoff to the oceans. This also stimulates phytoplankton growth, removing CO2 from the atmosphere as before. Thus, Gaia maintains a fairly constant climate as the sun heats up.

    Life has other influences over the chemistry of the planet: methane and ammonia exist in their present abundances because bacteria continually regenerate them by decomposing organic matter.

    Perhaps life regulates the physical and chemical environment of the planet so as to maintain suitable planetary conditions for the good of life itself. If so, then the planet can be thought of as a single, integrated, living entity with self-regulating abilities. This is the radical view that Lovelock and Margulis have espoused. It can be thought of as the "strong Gaian model."

4. Alternatives to the Gaia Hypothesis


  1. Oxygen: was first produced more than 3 billion years ago. Why did it take 2 billion years to build up? It took 2 billion years for all of the available reactive chemicals of the earth's surface, such as iron, to be oxidized to produce the sedimentary layers known as red beds. When it finally did build up, it forced then-existing life, which was poisoned by oxygen, to retreat to anaerobic (lacking oxygen) environments such as the bottom of swamps. Granted, this set the stage for the development of the ozone layer, which shields us from ultraviolet light, and permitted the evolution of aerobic life, but it was very self-sacrificing on the part of the major life forms existing some 2 billion years ago. This is hardly homeostatic (homeostasis: the ability or tendency of an organism or cell to maintain internal equilibrium by adjusting its physiological processes).

  2. Climate: geochemists have offered an alternative feedback mechanism for temperature control, which is accomplished inorganically [University of Michigan geochemists Walker, Hayes and Kasting]. As the sun warms, rains increase. Water in the atmosphere combines with CO2 to form a weak carbonic acid solution, and this could have weathered silicate minerals in the rocks on land. Formation of new sediments would store carbon, thus reducing its abundance in the atmosphere. Gaian advocates fight back by arguing that soil bacteria play a role in the weathering reaction. This is unresolved.

  3. The idea that climate and life influence one another is profoundly important. In some form or another, it has been recognized for a long time. Life and climate "grew up together" and influenced one another over most of earth history. But this is not to say that life somehow manages and self-optimizes its own environment. It is this idea -- the "strong form of Gaia" -- that is most controversial.

5. The Many Gaian Hypotheses

Jim Kirchner (see "suggested readings") argues that there are really many Gaian hypotheses.


  1. Influential Gaia, the weakest of the hypotheses, asserts that biota have a substantial influence over certain aspects of the abiotic world, such as temperature and the composition of the atmosphere.

    "The Gaia hypothesis.. states that the temperature and composition of the Earth's atmosphere are actively regulated by the sum of life on the planet" (Sagan and Margulis, 1983).

  2. Co-evolutionary Gaia asserts that the biota influence their abiotic environment, and that the environment in turn influences the evolution of the biota by Darwinian process.

    "The biota have effected profound changes on the environment of the surface of the earth. At the same time, that environment has imposed constraints on the biota, so that life and the environment may be considered as two parts of a coupled system" (Watson and Lovelock, 1983).

  3. Homeostatic Gaia asserts that the biota influence the abiotic world, and do so in a way that is stabilizing, by negative feedback linkages.

    "The notion of the biosphere as an active adaptive control system able to maintain the earth in homeostasis we are calling the 'Gaia' hypothesis" (Lovelock and Margulis, 1974).

  4. Teleological Gaia asserts that the atmosphere is kept in homeostasis, not just by the biosphere, but by and for the biosphere.

    "the Earth's atmosphere is more than merely anomalous; it appears to be a contrivance specifically constituted for a set of purposes" (Lovelock and Margulis 1974).

  5. Optimizing Gaia asserts that the biota manipulate their physical environment for the purpose of creating biologically favorable, or even optimal, conditions for themselves.

    "...it is unlikely that chance alone accounts for the fact that temperature, pH and the presence of compounds of nutrient elements have been, for immense periods, just those optimal for surface life. Rather, ... energy is expended by the biota to actively maintain these optima" (Lovelock and Margulis, 1974).

Kirchner argues that the weak forms of the hypothesis are not new and that the strong forms are not correct or not testable. He suggests that many people point to evidence for one of the weak forms of Gaia, and then go on to claim a stronger form.

6. Modeling Gaia

You can model feedbacks using the classic Gaia example of Daisyworld with Stella or using this interactive Java applet.  The latter is especially useful to get a first-order understanding of changing parameters.  The Stella model permit more sophisticated analysis.

7. Summary

  • The Gaia Hypothesis states that life on earth controls the physical and chemical conditions of the environment (the biotic controls the abiotic)

  • The hypothesis was formulated by James Lovelock and Lynn Margulis

  • The hypothesis points to stable conditions, such as oxygen levels and climate, as evidence that living organisms maintain a life-sustaining environment

  • The hypothesis has been defined and argued in numerous ways, and has as many critics as adherents. It is in need of more explicit formulation before it can be examined and tested as a true scientific theory.
  • Two models emerge:
    The model that life influences planetary processes (i.e., it has a substantial effect on abiotic processes) has become known as the weak Gaia hypothesis.  This model is widely supported.
    The original Gaia hypothesis, that life controls planetary processes (i.e., life created Earth's system), has become known as the strong Gaia hypothesis.  It is not widely accepted.


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