Introduction

The history of life has been shaped by great events that today are referred to as Major Transitions (Maynard Smith and Szathmary, 1995), or steps of macroevolu-tion, or, more simply, origins. The most important are the origins of: (1) genes; (2) proteins; (3) first cells; (4) eukaryotes; (5) embryos; (6) mind; and (7) language.

The feature that defines these great events is the appearance of new biological objects, and this raises immediately a fundamental question: "What is the mechanism that generates novelties in life?" It is no secret that today most biologists accept the conclusion of the Modern Synthesis, i.e. the idea that natural selection alone has the power to create evolutionary novelty. A different answer, however, has consistently come from embryology where a long-standing tradition has repeatedly claimed that the major biological novelties are generated by development and that natural selection can act on them only when they have been brought into existence.

This classical claim has not been been invalidated by the discoveries of Evolutionary Developmental Biology (EvoDevo), and recently it has been reproposed by Scott Gilbert (2006a,b) who has stressed that "the generation of novelty is the province of developmental biology." Gilbert's conclusion is based on two groups of arguments. The first is that we finally have the essential lines of a new theory of evolutionary variation (Arthur 2004; Callebaut and Rasskin-Gutman, 2005; Gilbert 2006a), a theory based on two preconditions (gene duplication and modularity) and four mechanisms of bricolage (heterotopy, heterochrony, heterotypy, and heterometry). The second group of arguments comes from the discoveries of developmental genetics that have falsified two major expectations of the Modern Synthesis (the alleged impossibility of deep genetic homology, and the claim that parallel evolution is proof of the creative power of natural selection).

These unexpected discoveries have been the starting point of EvoDevo, but Gilbert's interpretation of them is still a minority view in this field. Most supporters of EvoDevo regard them as proof that developmental biology has a very substantial contribution to make to evolutionary theory, but do not see any need to abandon the paradigm of the centrality of natural selection (Holland, 1999; Davidson, 2001; Gould, 2002; Wilkins, 2002; Carroll, 2005; Ruse, 2006).

Here I will try to show that Gilbert is right in challenging this paradigm and in claiming that there are two distinct mechanisms of evolution, but that he is wrong in saying that the second mechanism is embryonic development. If this were true, we would have to conclude that natural selection has been the sole mechanism of evolution for the entire period that preceded the origin of embryonic development, i.e. for about 3000 million years. More than that, if natural selection was the sole novelty-generating mechanism before the origin of embryos, it must have been natural selection that brought the first embryos into existence. And if the first steps toward development were taken by natural selection, why should the other steps have been different? When did the break occur between natural selection and embryonic development? Arguments like these make it difficult to accept the longstanding claim of embryology, and that is why the centrality of natural selection appears to be the only reasonable option that we are left with, even when we learn that recent discoveries have falsified some predictions of the Modern Synthesis.

There is, however, another solution to the problem of evolutionary novelty. It is the idea that there have been two distinct mechanisms of evolution throughout the whole history of life. The idea that evolution took place by natural selection and by natural conventions from the very first cells onwards (Barbieri, 1985, 2003). The second mechanism of evolution, in short, is based on natural conventions, i.e. on organic codes, and has been present on Earth since the origin of protein life because specific proteins cannot exist without a genetic code. In order to back up this new theory, however, we need to prove: (1) that many organic codes have appeared throughout the history of life; and (2) that organic coding is a mechanism of evolution that is distinct from natural selection. These are the key points that will be addressed here and to that purpose the paper has been divided into two parts: (1) the organic codes; and (2) the mechanisms of evolution.

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