To what extent has life governed environmental conditions.? Sound answers to this basic question are not forthcoming. There are two diametrically opposed possibilities, and all gradations between them.
One extreme viewpoint is that life is an inconsequential film that, historically, has had little impact on surface processes, at least until humans arrived (e.g. Holland 1984). It adapts as best it may through biological evolution, ducking and weaving to avoid the brunt of geological and cosmic forces, bowing to the whim of every volcanic eruption, sea-level change, and asteroid impact. This is a somewhat gloomy view of life's role on the Earth. Greg Retallack (1990, 1992) dubbed it the Ereban hypothesis, after Erebus, the Greek personification of darkness and the underworld. I preferred the Hadean hypothesis (after Hades, the Greek god of the underworld), in which life is seen to struggle painfully for existence amidst a geological and cosmic Hell (Huggett 1997a; Figure 9.1). The Hadean view sees the biosphere as a dynamic system capable of responding to changes in its environment, but only in a limited and essentially passive way. It paints the biosphere as a somewhat fragile system that is susceptible to permanent disruption.
At the other extreme, the Gaia hypothesis asserts that, shortly after if first appeared, life has been at the helm, exercising near total homeostatic control of the terrestrial environment (Figure 9.1). The brighter Gaian view of life's connection with the planet is named after Gaia, the Greek goddess of the Earth, daughter of Chaos, mother and lover of the sky (Uranus), the mountains (Ourea), and the sea (Pontus).
Between the extremes of Hades and Gaia lies a gamut of middle-of-the-road views expressing varying degrees of control over the terrestrial environment. Some commentators feel these are more reasonable than the extreme views, pointing out that life did not rapidly assume hegemony over Earth's resources after its first appearance, nor has it been wholly at the mercy of cosmic and geological forces (e.g. Retallack 1992). A report of a seminar at Green College, Oxford (Tickell 1996), in the mid-1990s suggested that mid-dle-of-the-roadism was gaining supporters, with many geoscientists subscribing to the view that the entire Earth does has lifelike properties, though it is not alive. This kind of thinking has produced the new subject of geobiology, which has spawned a journal of that name (see Knoll 2003). Geobiologists strive to understand the role of organisms in the Earth system, looking particularly at the manner in which evolutionary innovations and the population genetic process that mediate evolution have influenced the Earth's surface through its history (Knoll 2003).
Age (million years)
Figure 9.1 Views on the control wielded by life over the ecosphere. Source: After Huggett (1997a), partly adapted from Retallack (1992).
Age (million years)
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