We introduced the Gaia hypothesis briefly in chapter 6. According to this hypothesis, the physical environment alters life via natural selection, but life also alters (some would say "regulates") the physical environment. The original proponent of the idea, James Lovelock, observed that the
27 "Group selection theory", advanced by Trivers in 1971, has always been controversial and some writers, not least Richard Dawkins, are strongly antipathetic to it. Dawkins says that everything attributed to group selection can be accounted for by kin selection (behaving altruistically towards close relatives, which share a large percentage of your genome); kin selection theory was pioneered by Hamilton in 1964. Nevertheless some recent ecological evidence, surveyed for example by Sober and Wilson in 1998, is difficult to explain without recourse to group selection.
atmospheres of Mars and Venus are very close to chemical equilibrium, but the Earth's atmosphere is very far from equilibrium. The difference arises, according to Lovelock, because there is no life on Mars or Venus to alter the composition of the atmosphere. The composition of the Earth's atmosphere remains stable for very long periods, though its component molecules are rapidly exchanged with the planet's surface. The oxygen content is maintained at a level sufficient for the needs of large oxygen-dependent organisms such as ourselves, but not high enough to cause widespread fires, which would seriously disrupt life. Life keeps the atmosphere stable, yet far from chemical equilibrium.
Another argument in favour of Gaia is that purely geochemical explanations of the Earth's climate history seem inadequate. For example, the reflection of sunlight from the ice caps should lead to intermittent total glaciation of the planet. This seems to have happened only very occasionally, if at all. The near-constancy of the Earth's average surface temperature over extended periods is remarkable; the sun's power has increased by some 25% over the past two thousand million years. The only influence that can have buffered the planet against the expected gradual heating and intermittent global cooling, say the proponents of Gaia, is the presence of life.
Calcium liberated from silicate rocks by weakly acid rain reacts with carbon dioxide to form calcium carbonate (limestone or chalk). This process removes carbon dioxide from the atmosphere, making the rain less acidic. This in turn decreases the weathering of silicate rocks. These days, everyone knows that carbon dioxide is a greenhouse gas; as its levels in the atmosphere rise and fall, so does the temperature near the surface of the planet. So the rate of silicate rock weathering and limestone formation are linked to the surface temperature via atmospheric carbon dioxide levels. However, the processes just described are much too slow to account for the geological evidence. The involvement of organisms in removing atmospheric carbon dioxide by photosynthesis and regenerating it by respiration and decomposition accounts more credibly for the speed of geological events.
Contrary to what some critics have suggested, the Gaia hypothesis is consistent with the natural selection model of evolution. In fact, it extends the argument we discussed in the previous section (different species within an ecosystem influence one another). The growth rate of any population depends on environmental variables; life defines the tolerable limits of such variables. Natural selection determines which species dominate the environment at any given time. By definition, these dominant species exert the greatest effects on the environment. So the environment determines which species predominate, and the dominant species alter the environment. Organisms and environment are tightly coupled and co-evolve. If a "pro-Gaian" and "anti-Gaian" mutation were to arise simultaneously within a population, then the "pro-Gaian" ones would be preferentially selected. Computer simulations support this prediction.
Beyond doubt, the Gaia hypothesis is valid to this extent: life affects the environment just as the environment affects life. This principle helps to explain the Earth's geological and climatic history and leads to predictions that can be tested by simulation.
However, it is unwise to extrapolate the hypothesis too far. The influence of life might not always accelerate or retard geological ageing to a significant extent. Also, it is not clear that the whole Earth behaves homeostatically, as some proponents of Gaia would claim. There are no plausible grounds for considering the whole planet to be a unit of selection or a "superorganism", as some enthusiasts have proposed. The Gaia hypothesis is valuable, but we should not transmute it into mysticism.
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