Zachriel linked to an article entitled Exiting an RNA world; Nature Structural Biology 7, 5 – 7 (2000) doi:10.1038/71194; authored by Paul Schimmel & Shana O. Kelley. Snippets from the article are reproduced here for the purpose of commentary. You can read the full article as linked. From the article:
"In contemporary biology the connection between the RNA world and proteins is made by the aminoacylation of transfer RNAs. In this reaction, the rules of the genetic code are established by virtue of linking each amino acid with the tRNA that bears the cognate anticodon triplet. The code in turn makes template-directed protein synthesis possible."
Amino acid encoding codons have no value in the absence of a means of translating transcribed messages. Translation is effected through tRNA which links specific amino acids with appropriate anticodons attached to tRNA.
"Given the results obtained thus far, further selections and tinkering will probably yield substantial improvements "” molecules that come closer to having the catalytic activity of modern protein-based aminoacyl-tRNA synthetases. Of course, an additional challenge remains in the attainment of RNA-based amino acid activation reactions because the availability of reactive amino acid esters was likely to be low in the aqueous environment of the proposed RNA world."
Researchers have "tinkered" to find RNA catalysts that would mimick the function of aminoacyl-tRNA synthetases. Those protein catalysts are involved in attaching the correct amino acids to tRNA; a function that needs accounting for in a theorized RNA world. We are reminded of the importance of an adaquate supply of twenty amino acids- available as needed.
"In the RNA world, ribozymes that catalyzed aminoacylations may have progressed through a succession of intermediates (Fig. 2). These intermediates could have included those with an enhanced efficiency because they were aminoacylated."
OOL papers generally have at least one part like the foregoing where authors speculate as to what might have been and then state their views as to what was likely. An argument is made for a selection process. The RNA world commences with an RNA self-replicator. An indicator of the difficulty in securing supporting data is evident by a lack of details explaining the conditions in which a succession of intermediates is thought to have evolved. Presumably there is some sort of genome but what functions are specified by it and how is replication accomplished. Do the biological structures exist within a cellular membrane and if so what structures would be included within its boundaries. What process leads from a self-replicator to tRNA groups? Why should we assume prebiotic dynamics proceed in this direction?
The weakest part of the case for an RNA world lies in the theory behind it. The RNA world often invokes a selection process without specifying details as to replicating entities and a directional indicator. Why would enhanced efficiency be a determining factor indicating the outcome of a chemical process in the absence of some precursor cell? If such a cell existed where are the steps delineating the process of getting there? Why not instead describe the likelihood of theorized chemical reactions in terms of free energy of activation?