How plausible are the feedback mechanisms that need to be invoked to explain the climatic record?
The formation of life is usually imaged in terms of Darwin’s classical warm pond of organic soup, which requires a friendly climate. But there are other scenarios which are much less sensitive. One recently popular theory of abiogenesis postulates early life getting started at underwater volcanic vents (Chyba 1998; for a popular account, see Fortey (1997)). Since this one is driven entirely by volcanic energy, it is essentially insensitive to surface climate, within very wide limits — the oceans should neither boil away nor freeze solid to the bottom, but surface temperatures anywhere between –100 and +100 centigrades ought be ok. A frozen surface under a faint sun is thus no obstacle (Bada & Bigham & Miller 1994). An interesting present-day analogy is Jupiter’s moon Europa (Kerr 1997; McKinnon 1997; Carr et al 1998; Khurana et al 1998; Greenberg et al 1998), where it is speculated that life may exist in an ocean underneath a thick ice cover (Gaidos & Nealson & Kirschvink 1999), despi
