Sir: Even with the most optimistic of assumptions the emergence of a primitive living system from a suitably constructed primordial soup, including amino acids, nucleotides etc, is an exceedingly improbable process ("Water found by the light of a Jupiter moon", 15 August). For instance, calculations by Fred Hoyle and myself, and independently by Francis Crick, have led to estimates for the odds against the occurrence of life that can only be described as being superastronomical.
As long as the Earth was the only planet where life is found a critic of "cosmic life" can take refuge in the statement that a posteriori statistics are irrelevant. Even the most improbable events do indeed occur, the critic could say, in defence of the paradigm of Earth-centred life. And in defiance of Copernican philosophy one might even assert that this exceedingly improbable - well-nigh miraculous - event took place here on the Earth.
The discovery of life on at least one other planet would instantly rule out this line of argument, however. Identical, superastronomically improbable transitions from non-life to life could not have happened independently on two separate planetary objects. It is immeasurably more probable that the two sites were either cross-infected, or co-infected from a common cosmic source. In either case the firm requirement is for microbial life to be transferred across astronomical distances. Panspermia is vindicated and the Earth-centred primordial soup would seem to be ruled out.
N C WICKRAMASINGHE