Responding to Stephen Fletcher's Views in the Times Literary Supplement on the RNA World
To the Editor
The Times Literary Supplement
The RNA World
Having with indignation rejected the assumption that the creation of life required an intelligent design, Mr Fletcher has persuaded himself that it has proceeded instead by means of various chemical scenarios.
These scenarios all require intelligent intervention. In his animadversions, Mr Fletcher suggests nothing so much as a man disposed to denounce alcohol while sipping sherry.
The RNA world to which Mr Fletcher has pledged his allegiance was introduced by Carl Woese, Leslie Orgel and Francis Crick in 1967. Mystified by the appearance in the contemporary cell of a chicken in the form of the nucleic acids, and an egg in the form of the proteins, Woese, Orgel and Crick argued that at some time in the past, the chicken was the egg.
This triumph of poultry management received support in 1981, when both Thomas Cech and Sidney Altman discovered the first of the ribonucleic enzymes. Their discoveries moved Walter Gilbert to declare the existence of an RNA world in 1986. When Harry Noeller discovered that protein synthesis within the contemporary ribosome is catalyzed by ribosomal RNA, the existence of an ancient RNA world appeared "almost certain" to Leslie Orgel.
And to Mr Fletcher, I imagine.
If experiments conducted in the here and now are to shed light on the there and then, they must meet two conditions: They must demonstrate in the first place the existence of a detailed chemical pathway between RNA precursors and a form of self-replicating RNA; and they must provide in the second place a demonstration that the spontaneous appearance of this pathway is plausible under pre-biotic conditions.
The constituents of RNA are its nitrogenous bases, sugar, and phosphate. Until quite recently, no completely satisfactory synthesis of the pyrimidine nucleotides has been available.
The existence of a synthetic pathway has now been established. (Matthew W. Powner et al, "The Synthesis of activated pyrimidine ribonucleotides in prebiotically plausible conditions," Nature 459, 239--242, 2009).
Questions of pre-biotic plausibility remain. Can the results of Powner et al be reproduced without Powner et al?
It is a question that Powner raises himself: "My ultimate goal," he has remarked, "is to get a living system (RNA) emerging from a one-pot experiment."
Let us by all means have that pot, and then we shall see further.
If the steps leading to the appearance of the pyridimines in a pre-biotic environment are not yet plausible, then neither is the appearance of a self-replicating form of RNA. Experiments conducted by Tracey Lincoln and Gerald Joyce at the Scripps Institute have demonstrated the existence of self-replicating RNA by a process of in vitro evolution. They began with what they needed and purified what they got until they got what they wanted.
Although an invigorating piece of chemistry, what is missing from their demonstration is what is missing from Powner's and that is any clear indication of pre-biotic plausibility.
I should not wish to leave this discussion without extending the hand of friendship to every party.
Mr Nagel is correct in remarking that Mr Fletcher is insufferable. Mr Walton is correct in observing that the RNA world is imaginary. And Mr Fletcher is correct in finding the hypothesis of intelligent design unacceptable.
He should give it up himself and see what happens.