In The Beginning

One of Daniel Dennett's favourite cautions is to be careful what you infer from failure of imagination:

"Here I had committed the sin I'd so often found in others: treating a failure of imagination as an insight into necessity"

(Daniel C. Dennett, “Can Machines Think?”, in Michael G. Shafto (Ed), How we Know, Harper and Row, San Francisco, 1985, pp. 121–145).

"Philosophers' Syndrome: mistaking a failure of imagination for an insight into necessity".

(Daniel C. Dennett, Consciousness Explained, Little, Brown and Company, Boston, 1991, p. 401).

"the philosopher's fundamental foible: mistaking a failure of imagination for an insight into necessity".

(Daniel C. Dennett, Darwin's Dangerous Idea: Evolution and the Meanings of Life, Simon and Schuster, New York, 1995, p. 175).

"But Descartes's almost articulated argument, like the more often discussed versions of the Argument from Design, mistakes a failure of imagination for an insight into necessity".

(Daniel C. Dennett, "Descartes's Argument from Design", in The Journal of Philosophy, Vol. 105, No. 7, July 2008, pp. 333–345).

A beautiful example of this danger is provided by a landmark paper just published in Nature: Matthew W. Powner, Béatrice Gerland and John D. Sutherland, "Synthesis of activated pyrimidine ribonucleotides in prebiotically plausible conditions", in Nature, Vol. 459, No. 7244, 14 May 2009, pp. 239-242. Jack W. Szostak summarises the discovery as follows:

For 40 years, efforts to understand the prebiotic synthesis of the ribonucleotide building blocks of RNA have been based on the assumption that they must have assembled from their three molecular components: a nucleobase (which can be adenine, guanine, cytosine or uracil), a ribose sugar and phosphate. Of the many difficulties encountered by those in the field, the most frustrating has been the failure to find any way of properly joining the pyrimidine nucleobases — cytosine and uracil — to ribose. The idea that a molecule as complex as RNA could have assembled spontaneously has therefore been viewed with increasing scepticism. This has led to a search for alternative, simpler genetic polymers that might have preceded RNA in the early history of life. But Powner et al. revive the prospects of the 'RNA first' model by exploring a pathway for pyrimidine ribonucleotide synthesis in which the sugar and nucleobase emerge from a common precursor. In this pathway, the complete ribonucleotide structure forms without using free sugar and nucleobase molecules as intermediates. This central insight, combined with a series of additional innovations, provides a remarkably efficient solution to the problem of prebiotic ribonucleotide synthesis.

Later, he concludes:

Of course, much remains to be done. We must now try to determine how the various starting materials could have accumulated in a relatively pure and concentrated form in local environments on early Earth. Furthermore, although Powner and colleagues' synthetic sequence yields the pyrimidine ribonucleotides, it cannot explain how purine ribonucleotides (which incorporate guanine and adenine) might have formed. But it is precisely because this work opens up so many new directions for research that it will stand for years as one of the great advances in prebiotic chemistry.

And so one more complex structure falls prey to nature's cunning. For commentary, see here and here.