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ScienceWeek
EARTH SCIENCE: ON GAIA THEORY
The following points are made by James Lovelock (Nature 2003: 426:769):
1) 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.
2) 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.
3) Although the concept of a live Earth is ancient, Isaac Newton (1642-1727) was the first scientist to compare the Earth to an animal or a vegetable. James Hutton (1726-1797), Thomas Henry Huxley (1825-1895), and Vladimir Vernadsky (1863-1945) expressed similar views but, lacking quantitative evidence, these earlier ideas remained anecdotal. In 1925 Alfred Lotka (1880-1949) 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.
4) Although welcomed by atmospheric scientists, Earth scientists were cautious. Biologists, especially Ford Doolittle and Richard Dawkins, argued strongly that global self-regulation could never have evolved, as the organism was the unit of selection, not the biosphere. In time I realized that they were right -- but still I thought, something keeps the Earth habitable. In 1981 I composed a model of dark- and light-colored plants that competed for growth on a planet in progressively increasing sunlight. My intention was not to make a blueprint for the Earth, but a model to show that Gaia is consistent with natural selection. This "Daisyworld" regulated its temperature close to that fittest for plant growth and -- unusually for an evolutionary model made from coupled differential equations -- it was stable, insensitive to initial conditions and resistant to perturbation.
5) Daisyworld is darwinian, but the evolution of the organisms and the evolution of temperature proceed as a single, coupled process. The model was much criticized, but so far has resisted falsification. It was easy to show that Daisyworld tolerates "cheats" -- daisies that grow but offer nothing towards self-regulation. Other critics claimed that daisies would adapt to changing temperature and therefore simply track temperature change, not regulate it. But the restraining function connecting growth with temperature is not negotiable; chemistry, not biology, sets its constants.
6) By the end of the 1980s there was sufficient evidence, models and mechanisms, to justify a provisional Gaia theory. Briefly, it states that organisms and their material environment evolve as a single coupled system, from which emerges the sustained self-regulation of climate and chemistry at a habitable state for whatever is the current biota. In its present form, Gaia theory reconciles current thinking in evolutionary biology with that in evolutionary geology. It extends, not contradicts, Darwin's vision, just as relativity enhances, not denies, Newtonian physics. The theory is provisional, but provides an intellectual habitat where understanding of the Earth can evolve and grow. Perhaps its greatest value lies in its metaphor of a living Earth, which reminds us that we are part of it and that human rights are constrained by the needs of our planetary partners.(1-5)
References (abridged):
1. Kump, L. R., Kasting, J. F. & Crane, R. G. The Earth System 2nd edn (Pearson Prentice Hall, Upper Saddle River, New Jersey, 2004).
2. Hamilton, W. D. & Lenton, T.M. Ethol. Ecol. Evol. 10, 1–16 (1998).
3. Lenton, T. M. Nature 394, 439-447 (1998).
4. Lovelock, J. E. The Ages of Gaia (Oxford Univ. Press, Oxford, 2000).
5. Margulis, L. The Symbiotic Planet (Phoenix, London, 1999).
Nature http://www.nature.com/nature
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ON GAIA AND NATURAL SELECTION
Considering a global system as complex as the planet Earth and its resident biological entities, there are many possible schemes, some more metaphorical than others, for organizing observations and predicting future events. In recent years, The Gaia hypothesis, first formulated by James E. Lovelock in 1979, has emerged as a possible conceptual framework for studying the interaction of the environment of the surface of the Earth and Earth's biota. The essential aspect of the Gaia hypothesis is the consideration of feedback mechanisms whose consequence is self-regulation of the environment-biota global system with an emphasis on the importance of the biotic component in the physical history of the planet.
The Gaia hypothesis has its severe critics, as evidenced in the following view of Tjeerd H. Van Andel (University of Cambridge, UK) (1994) (Note #1): "The conflict between accepting what science teaches us and what the human heart would like to believe is well illustrated by James Lovelock's Gaia concept that places life in charge of the functioning of our planet. It is a lovely thought, a tempting one too, because it is a form of religion and the human soul requires the comfort of a guided Universe; it needs religion. Alas, it is also unnecessary, because the world as it was, has evolved, and now exists, is not explicable. It is merely very complex, and life plays a role in it, but not the main one."
The above evaluation notwithstanding, the effort to find a workable scheme to generate understanding of environment-biota interactions continues.
The Gaia theory was named for Gaia, the Greek Earth-mother goddess.
The following points are made by Timothy M. Lenton (Nature 1998 394:439):
1) Organisms alter their material environment, and their environment constrains and naturally selects organisms. This connection indicates feedback between life and its environment.
2) The Gaia theory proposes that organisms contribute to self-regulating feedback mechanisms that have kept Earth's surface environment stable and habitable for life, and the theory seeks to explain these mechanisms and how they arise.
3) Natural selection, acting on faithful replication of inherited variation, determines that the organisms that dominate are the ones that leave the most descendants. Together, natural selection and Gaia pose a puzzle: How can self-regulation at the planetary level emerge from natural selection at the individual level?
4) The author attempts to address this puzzle by focusing on the feedbacks to biospheric growth and selective pressures that can arise from environment-altering traits of the biota. Land ecosystems and marine phytoplankton are discussed in terms of several models. The author concludes: "The implications may be far-reaching; simple principles suggest that environmental regulation can emerge at levels from the individual to the global. Natural selection is seen as an integral part of Gaia, and Gaia theory also has something to offer evolutionary biology. Gaian models suggest that we must consider the totality of organisms and their material environment to fully understand which traits come to persist and dominate."
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Notes:
Note #1: T. H. Van Andel: New Views on an Old Planet: A History of Global Change. Cambridge University Press 1994, p. 402. More information at: http://www.amazon.com/exec/obidos/ASIN/0521447550/scienceweek
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