WHY NO NEW PHYLA AFTER THE CAMBRIAN - GENOME AND ECOSPACE HYPOTHESES REVISITED

Considering the implications of the phylogenetic tree, the fossil record suggests that all metazoan phyla had originated by the close of the Early Cambrian. Two hypotheses for the ensuing restriction on, body-plan origination are 1) that genomes have become modified so as to preclude the origin of major morphological novelty; and 2) that the environment has become so widely occupied by numbers of distinctive lineages that any evolutionary opportunities are quickly exploited by existing clades. The burgeoning array of findings on the molecular basis of development suggests that the genome hypothesis is unlikely to explain, the restriction on evolutionary novelty. Body plans are specified through a system of pattern-formation and selector genes that was in place before the Cambrian explosion; although this system has been modified within different clades as they have diverged, there are no indications that the creative ability of regulatory genomes has been impaired. A model of ecospace occupation involving evolutionary preclusion and access appears to be consistent with the data as now understood.