There are a number of ways of considering the origin of the genetic code, which can basically be grouped into two broad viewpoints: (i) an early origin, perhaps even related to the origin of life (Mackinlay, 1982); and (ii) a late origin (Di Giulio, 1998) characterising only the later phases of the origin of life or, rather, the evolution of the last universal common ancestor (LUCA) (Di Giulio, 2001).

It seems more sensible to me to consider genetic code origin as representing the terminal phases of the evolution of the origin of protein synthesis and having nothing to do with the origin of life. Therefore, its origin should not be early on. This is justified by the fact that I am unable to find a plausible reason for the origin of the genetic code unless in an already complex system, and this evolutionary stage seems to be very late on in the origin of life (Di Giulio, 1998, 2001). Furthermore, in order to trigger a selective pressure that may eventually result in the origin of the genetic code, I think that certain circumstances need to exist.

Laboratory of Molecular Evolution, Institute of Genetics and Biophysics 'Adriano Buzzati Traverso', CNR, Via P. Castellino, 111, 80131 Naples, Italy, e-mail: [email protected]

M. Barbieri (ed.), The Codes of Life: The Rules of Macroevolution. © Springer 2008

Firstly, a permanent necessity to evolve, and hence codify, something truly important, such as the evolution of enzymatic catalysis because without catalysis there is no life. Secondly, catalysis itself must have been performed, at least in some of its evolutionary stages, by molecules that have something to do with genetic material because, otherwise, the link to the genetic code would be difficult to achieve. In this case, the evolution of enzymatic catalysis may even have come to a halt at a rudimental stage, perhaps leading inevitably to the extinction of life itself or at least severely limiting its complexity. In other words, the replicating molecules must, at some evolutionary stage, have had something to do with catalysis-performing molecules because otherwise the link between these two worlds would have been difficult to achieve. All the above introduces the topic discussed in the following section.

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