annelids was similar, with a hydrostatic hemocoel; these forms are probably among trace makers of the late Precambrian. Cell-phenotype numbers in living phyla, and a model of cell-phenotype number increase, suggest an origin of metazoans near 600 my ago, followed by a passive rise in body-plan complexity. Living phyla appearing during the Cambrian explosion have a Hox/HOM gene cluster, implying its presence in the common ancestral trace makers. The explosion required a repatterning of gene expression that mediated the development of novel body plans but evidently did not require an important, abrupt increase in genomic or morphologic complexity.

I thank Dave Jablonski (University of Chicago) and Allen Collins and Chris Meyer (University of California, Berkeley) for valuable discussions and reviews of the manuscript, and Clint Turbeville for insights into coelom development. This research was based on work supported by Grant EAR-9196068, National Science Foundation, and by Faculty Research grants, University of California, Berkeley.

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