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The fluxes of gases to the present atmosphere compared with those expected for an abiotic Earth (after Lovelock 1989).

Lovelock's insight led to a radically new explanation of Earth's habitability for the past 3 billion years (now accepted to be at least 3.5 billion years based on fossil evidence). This habitability was not just "dumb luck," but rather a result of continuous biotic interaction with the other components of the biosphere, the atmosphere, ocean, and soil/upper crust. The requirements of habitability include favorable temperatures, ocean salinity, and—at least for the past 2 billion years—atmospheric oxygen levels for aerobes. In Lovelock and Margulis's early papers, we find a formulation of Gaia as a homeo-static system:

From the fossil record it can be deduced that stable optimal conditions for the biosphere have prevailed for thousands of millions of years. We believe that these properties of the terrestrial atmosphere are best interpreted as evidence of homeostasis on a planetary scale maintained by life on the surface (Lovelock and Margulis 1974a).

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 andMargulis 1974b).

What are optimal conditions? Optimal for maximum productivity of ecosystems, the global biota? Optimal for the persistence of planetary biota, but which components? Is optimality to be measured in number of species? (If so, on the present Earth beetles apparently win out.) Did the anaerobic pro-caryotes of the Archean optimize atmospheric conditions for their successors, the aerobes? Optimality is a problematic concept at the very least.

After the publication of Lovelock's first book (1979), homeostatic Gaia came under heavy attack in the 1980s primarily from staunch neodarwinian biologists (Doolittle 1981, Dawkins 1982, Maynard Smith 1988). They objected to the concept of life optimizing its external conditions by natural selection because the biosphere is a single entity "competing" against no other (see discussion in Barlow and Volk 1992a). Furthermore, "Gaia, as a cybernetic system, must have mechanisms for sensing when global physical and chemical parameters deviate from optimum, and mechanisms for initiating compensatory processes which will return those parameters to acceptable values (negative feedback)" (Doolittle 1981).

In response to such criticism, Watson and Lovelock (1983) developed the Daisyworld model, an attempt to demonstrate the possibility of planetary surface homeostasis without invoking natural selection. This model in its simplest form assumes dark and light daisies populating a planet, subject to a steadily rising energy flux from outside (as the Earth/sun couple). The daisies differ only in their reflectivity (albedo) of incoming radiation, with the same growth and death rates. The result is that the planetary temperature significantly stabilizes as the incoming energy flux increases, an outcome of the "thermostat" set up by the successive expansion of first dark then light daisies, each affecting the planetary albedo (figure 1-3). Lovelock (1989) and others (Saunders 1994) have followed up this original model by making the "ecosystem" more complex (e.g., adding predators, shades of daisies) and more realistic controls on heat flow between regions on the hypothetical planet Daisyworld. The model results appear to strengthen the hypothesis that homeostasis, at least by albedo modification, would result from plausible biotic physiology, without natural selection operating to guarantee optimization.

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