Postchemical evolution

The first phase corresponds to classical chemical evolution, but the second one is more difficult to define, because it is no longer chemical evolution but not yet biological evolution. It is however necessary to characterise it, and to this purpose we can give it the name of postchemical evolution. Before the origin of life, in other words, there must have been two evolutionary stages that were temporally and conceptually distinct one of chemical evolution and the other of postchemical evolution. 

Such a distinction is important because it gives us a criterion for a better evaluation of the origin-of-life theories. The solutions proposed by Sidney Fox or Wachtershauser, for example, are exclusively theories of chemical evolution, and tell us nothing about postchemical evolution. It would be wrong to criticise them for this, but it would also be wrong to say that, if they explain chemical evolution, they also explain postchemical evolution and therefore the origin of the cell. 

The concept of postchemical evolution, in conclusion, allows us to realise that there is another important dichotomy in the origin of life field. In addition to the distinction between metabolism-first and replication-first theories, it is necessary to distinguish between theories of chemical evolution and theories of postchemical evolution. 

Collectively, these facts strongly suggest that RNAs had a leading role in what here has been called postchemical evolution. It must be underlined, however, that RNAs are sophisticated, evolutionarily advanced molecules (Miller, 1987 Joyce, 1989 Orgel, 1992), and all the above facts do not allow us to conclude that they were also leading players in the earlier phase of chemical evolution. 

We conclude that the replication paradigm has not been able, so far, to account for chemical evolution, but could be valid for postchemical evolution, and this, it will be remembered, is also Dyson s hypothesis (metabolism first, replication second). Let us examine therefore the evolutionary potential of primitive vesicles containing RNAs that could behave both as genes and enzymes. 

We are bound to conclude that the replication paradigm does not offer a plausible model even for postchemical evolution. Of course we cannot exclude that future discoveries might modify such a conclusion, but it would be necessary to discover, among other things, that primitive ribozymes were making replication errors comparable to those of protein enzymes, and this is extremely unlikely. 

For the time being, therefore, the only reasonable conclusion is that a true replication mechanism appeared only at the end of precellular evolution, when the first cells came into being. Both chemical evolution and postchemical evolution, in other words, had to take place with metabolic systems that were able to tolerate errors, because only in this way could they be immune to the replication catastrophes. 

In 1981 I proposed the first model of postchemical evolution with the ribotype theory on the origin of life, and with the concept of ribotype, a term that indicates all RNAs and ribonucleoproteins of any organic system (Barbieri, 1981). Since ribonucleoproteins are advanced compounds, the name rihosoids was introduced to indicate all molecules made of RNA, or RNA and peptides, and the ribotype was also defined as the collective of all rihosoids of an organic system. 

This is the novelty that characterised the first part of postchemical evolution, and had at least one important consequence the polymerising ribosoids allowed for the first time the production of peptides and small proteins inside the system, with endogenous syntheses, instead of importing these molecules from the outside. And this switch from exopoiesis to endopoiesis was an essential prerequisite for the development of a true autopoiesis. 

In addition to polymerising ribosoids, precellular systems were producing many other types of ribosoids, and, for statistical reasons, most of these were devoid of any metabolic value. A few, however, could have more interesting properties and behave, for example, like ribozymes or transfer-like RNAs. The first part of postchemical evolution was therefore a simple continuation of the metabolic processes of chemical evolution, with the difference that precellular systems were now carrying RNAs in their interior, which means that both the players and the rules of metabolism were slowly changing. 

Let us now come to the second part of postchemical evolution, the stage that was destined to lead to the origin of the first cells. It is in this stage that we must look for an answer to the problem that the replication paradigm has been unable to solve how did primitive systems manage to increase their complexity without being destroyed by error catastrophes The ribotype answer is based on three points. 

This then is the solution of the ribotype theory in order to avoid the error catastrophes in the journey toward exact replication, it was necessary to have high molecular weight protoribosomes, and the production of these ribosomes for an indefinite number of generations was possible, before exact replication, because ribosoids could achieve it with processes of self-assembly and quasi-replication. The development of high-molecular-weight protoribosomes took place during postchemical evolution, simply because all necessary conditions existed in that period, and the development could be realised with processes that were both natural and primitive. 

We conclude that, during postchemical evolution, what was taking place was not only a development of metabolic structures, but also an evolution of coding rules, of natural conventions. The true mechanism of postchemical evolution, in other words, was not genetic drift alone, but a combination of drift and natural conventions. To the classical concepts of evolution by genetic drift and by natural selection, we must add therefore the concept of evolution by natural conventions. 

This tells us that chemical evolution was really different from postchemical evolution. In the course of chemical evolution, the jump of primitive metabolic systems from chaos to order was only a question of statistical probability and energy conditions. During postchemical evolution, instead, a new type of antichaos appeared, an order that was based on conventional rules of correspondence between two independent molecular worlds, and it was from these first natural conventions that the genetic code finally emerged. 

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