Granted at least one planet somewhere in the galaxy (apart from our own) that meets the necessary criteria, how likely is it that life did begin there -and continued to flourish and evolve? This brings us back to the likelihood of the origin of life, which we discussed in chapter 14. If life on Earth was highly improbable then it is equally improbable everywhere. In other words it has probably not happened, since - as we have just argued - the supply of suitable planets is likely to be limited. Conversely, if there is or has been life elsewhere in the universe, then the origin of life on Earth must have been probable. Therefore, to consider the origin of life on Earth unlikely is, logically, to doubt the probability of life elsewhere in the universe. However, if we accept that extraterrestrial life is likely, then we must also believe that life on Earth was probable. If life on Earth was probable then it originated several times, only for the process suddenly to become impossible and for all but one lineage of organisms to be wiped out. For this reason, the search for extraterrestrial life, life with an origin distinct from Earth's, is justified: it throws light on one of the major issues that we raised In chapter 14. (Strictly speaking, this argument is valid only for protein and nucleic acid based life similar to ours. We cannot extend the reasoning to life with a completely alien chemistry - life without proteins and nucleic acids, say, or even without carbon. Since we cannot imagine such an alien biochemistry, we cannot conjecture what the conditions for it might be or how likely those conditions are.)
Suppose there is life on planets circulating other stars, and suppose it is protein and nucleic acid based. Will it evolve in a way comparable to ours? Genetic change and natural selection would surely be inevitable, so evolution must occur. But would ever-greater complexity be inevitable, as it seems to be on Earth? Our answer depends partly on whether we side with Kauffman or Gould: does evolution select among a limited though changing choice of patterns (Kauffman) or is it wholly contingent, patternless, a matter of pure luck (Gould)?
A case can be made in favour of progressive increase in complexity: in any developed ecosystem, extreme interdependence is highly likely among at least some species pairs; so symbiosis will probably result, leading to more complex organisms (chapter 13). But this is not a watertight argument. If symbiosis does not happen, would more sophisticated organisation still emerge because of the mathematical properties of complex adapative systems, fuelling an evolutionary trend towards increasingly complex organisms? It is not easy to imagine a plausible biological mechanism for this, other than symbiosis. Therefore, the emergence of organisms comparable to eukaryotes, and the subsequent emergence of multicellularity, is a possible but not inevitable (or even very likely) scenario for other worlds. If there is life like ours elsewhere, it has probably remained a prokaryotic enterprise.
Again, we cannot extrapolate this argument to life with an alien chemistry because it might have no analogues of "prokaryote", "eukaryote", "multicellularity", and so forth. The very notion of "evolution" might have to be revised in that situation. The bounds of reasonable speculation are exceeded.
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