Habitability for life is defined by the constraints of temperature, pH, and other physical and chemical parameters. The presently estimated upper temperature limit for life (for viable growth) is about 125°C, the limit for presently known hyperthermophiles. Life at this temperature is only possible at pressures sufficient to keep water liquid. Some believe hyperthermophiles may be discovered that are viable at a somewhat higher temperature (150°C), the apparent upper limit for protein stability sufficient to make metabolism possible (Brock et al. 1994). The lower temperature limit for growth is a few degrees below 0°C (saline water is still liquid). The origin of life and its persistence as thermophiles below ice cover in hydrothermal vents deeper in the crust might occur on an ice-covered Earth; one could also imagine the emergence of lower temperature forms, at least microbes.
These limits define the upper and lower temperature boundaries for "life as we know it" (one could speculate about exotic biochemistries in alien biospheres; see Feinberg and Shapiro 1980). The upper habitability limit for life was plausibly established between the accretion of the Earth some 4.5 billion years ago and the end of the late bombardment of Earth by leftover debris from planetary accretion some 3.7 to 3.8 billion years ago.
Recently, several molecular biologists suggested that the origin oflife took place around 4.2 Ga, based on the considerable complexity evident in the earliest fossil record (i.e., probable cyanobacteria by 3.5 Ga), as well as detailed arguments related to molecular phylogeny (see chapter 8). Large impacts may have even sterilized the surface (Oberbeck and Mancinelli 1994), driving the first life back into hydrothermal vents in the crust, the birthplace of hyperthermophiles (see chapter 8). In any case, the abiotic history of the solar system created the first window ofopportunity for life to emerge. Most origin of life researchers think this happened very fast once survival of the earliest life form was possible. Because the luminosity of the sun steadily increased according to the standard theory (see chapter 3), if the Earth at 3.8 Ga contained a pressure-cooker atmosphere, with a surface temperature around 85°C, then were it not for the removal ofatmospheric carbon dioxide by the carbonate-silicate cycle the temperature might have exceeded the habitability limit early in Earth history. Conversely, as Caldeira and Kasting (1992a) have shown, were it not for very warm conditions early on, the Earth might have plunged into an irreversible global ice age as a result of carbon dioxide cloud condensation raising the planetary albedo (although this mechanism has been recently questioned; Forget and Pierrehumbert 1997).
To jump some 6 billion years forward (1-2 billion years from now), the upper habitability limit will again be reached as atmospheric carbon dioxide level drops to zero, with a water greenhouse inexorably raising surface temperature (Caldeira and Kasting 1992b). Thus, 6 billion years of habitability with respect to surface temperature is:
1. A lucky accident [invoking the anthropic principle here, as Doolittle (1981) did, we are here to recognize this history, because "only a world which behaved as if Gaia did exist is observable because only such a world can produce observers") or
2. A product of deterministic self-regulation of the Earth's surface system.
This second alternative might claim that the silicate-carbonate biogeochem-ical cycle, the fundamental biospheric self-regulator of temperature, made possible continuous habitability from 4.2 Ga to the future end of the biosphere (a caveat is in order here: some researchers have suggested that at least surface life was reinvented several times in the early Precambrian). However, it is important to distinguish here the habitability for life, and habitability for low temperature life, or at least for complex life. The abiotic boundary conditions may well have guaranteed continuous habitability for thermo-philes for 6 billion years, but complex life may have required biotically mediated cooling via the geophysiological climatic stabilizer.
A lucky accident explanation is ultimately testable by the discovery of alien biospheres and their survival probabilities and dynamics. But an understanding of the latter for our own biosphere could conceivably rule out the lucky accident, just as origin oflife research now points to its near inevitability as a phenomenon of self-organization (Kauffman 1993, 1995).
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