Frozen Evolution. Or, that’s not the way it is, Mr. Darwin. A Farewell to Selfish Gene.

The Cambrian explosion is the rapid origination of probably all extant (and also many other already extinct) metazoan phyla around 545 million years ago. All of the basic architectures of animals were apparently established by the end of the Cambrian explosion; subsequent evolutionary changes, even those which allowed animals to move out of the sea onto land, involved only modifications of those basic body plans. Most probably, not only the general diversity of metazoan body plans, but also the diversity within particular phyla, reached its maximum within 10-15 million years of the Cambrian, and decreased throughout the following 500 million years. The existence of the Cambrian explosion is in accordance with the predictions of the frozen plasticity theory. At the beginning of the evolution of the metazoan clade, many traits, even those which determine the basic architecture of animal body plans, had the capacity to become plastic during peripatric speciations in many metazoan lineages. For an explanation, see also the question: “Why does the variability of species decrease with the age of the phylogenetic line and why does the maximal diversity (more correctly, the maximal disparity) of phylogenetic lines occur early in the history of the lines”. Therefore, a rather radical remodeling of body architecture as well as the origin of new body plans in response to particular selection pressures, were possible in the early stages of metazoan evolution. Through time, more and more traits came to be permanently frozen. Most probably, different traits would lose the capacity to turn plastic in differing successions in particular phyla. Therefore, anagenetic potential faded, and adaptation to new environmental conditions came to be based on modification of existing body plans rather than on building new ones.