The Cambrian singularity

Paleontology has shown that the history of life has been full of adaptive radiations, processes in which an ancestral taxon gave rise to descendant taxa which diverged by adapting to different environmental conditions. Some adaptive radiations have been explained with the mechanism of phyletic gradualism, while others are better described by punctuated equilibria, but in all cases they are classical processes of adaptation to the environment by natural selection. 

The temptation to declassify the Cambrian explosion, and to assimilate it to processes that can easily be accounted for, is strong, but leads to an unbridgeable contradiction between experimental data and theoretical previsions. The only reasonable conclusion, therefore, is that the Cambrian explosion was not an adaptive radiation, i.e. it was not a simple process of adaptation to the environment, but something very different. 

Such a conclusion is directly suggested by the very characteristics of the explosion. All adaptive radiations that came after the Cambrian have never modified the body plans, while the Cambrian explosion was characterised precisely by modifications of those plans. And, in a similar way, no adaptive radiations have ever changed the phylotypic stage of developing embryos, while the Cambrian explosion did precisely that. 

The message that nature herself appears to be sending us is that the Cambrian explosion was a rare event in the history of life, comparable perhaps only to the origin of life or to the origin of the mind. And what is so special about these rare episodes of macroevolution is the appearance of biological characteristics which have never been changed ever since. The mechanism that we are looking for, in conclusion, must explain precisely why the Cambrian explosion was so different from a normal adaptive radiation, even if this means that we cannot explain it with classical mechanisms. 

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