Evolution of the code

The genetic code is one of the most universal characteristics of life, and even its rare variant forms, that do exist in some micro-organisms, are different only in a few minor details. Such extraordinary uniformity means that the rules of the code have never been changed during the history of life, and go back all the way up to the very origin of the cell. Which is understandable, because a change in the code would change the structure of all proteins, and the entire system would collapse.
A full functioning code, however, emerges only when all its rules are present, and we need therefore to understand what existed before that point. Can we conceive half a code or a quarter of a code? And what sense would half a code have made before the origin of life, when there was no exact replication to pass its rules on?
The simplest way of answering these questions is by discussing a thought experiment, a highly idealized example that allows us to focus on the essential points of the problem. Let us imagine a primitive system which had simplified versions of the three main protagonists of protein synthesis, i.e. preribosomes, transfer-like RNAs and messenger-like RNAs. The messenger-like molecules were not transporting any message, and were mainly random linear sequences of nucleotides, but could still have a role to play. We have seen that most metabolic reactions are more likely to occur on a surface than in space, and the same thermodynamic arguments tell us that they are even more likely to occur along a line than on a surface. A linear chain of RNA was allowing frequent encounters of preribomes and transfer-like RNAs, and amino acids could be brought close together more easily on a line than on a surface. Thermodynamics, in other words, was favouring systems where peptide bonds were made by preribosomes and tRNAs attached to linear chains of RNA.