developmental steps that characterize the phyla, evolved after the pattern formation and selector genes were assembled. The Cambrian explosion in metazoan body plans may not have involved any great expansion of the gene regulatory apparatus but probably could not have occurred if the regulatory systems were not already sophisticated, and thus able rapidly to create novel morphologies, as the morphological innovations were evidently achieved through modes of regulatory evolution (Britten and Davidson, 1971; Valentine and Campbell, 1975; Jacobs, 1990; Valentine, 1994).

External Factors. The abrupt appearance of higher taxa in the Cambrian has stimulated a search for possible environmental changes that might have produced this evolutionary outburst; there has been no shortage of suggestions (Valentine et al., 1991; Signor and Lipps, 1992). However, the geological record has not revealed unequivocal evidence of unique Cambrian events that might be held accountable. One of the more common suggestions has been that oxygen levels rose in the late Precambrian to values that could sustain metazoans and thus permitted evolution to produce increasingly active organisms through time. Recent studies have indeed revealed series of late Precambrian—Cambrian geochemical excursions that can be plausibly interpreted as involving CO2 and oxygen levels and as being associated with the biogeochemical cycling of the time (Knoll, 1994).

It is not clear that an external trigger was needed to produce the Cambrian explosion. A continuous expansion of an already moderately complex fauna as various lineages acquired evolutionary access to broadening arrays of marine habitats, with the resulting enhancement of ecological interactions within that fauna, may be all that was required to produce the record we have, insofar as the origin of phyla is concerned. However, continuing geochemical and other studies will surely lead to a better understanding of the nature of environmental change during the Cambrian explosion, and then we shall be better able to judge the possible role of physical triggers.

Summary

A broad variety of body plans and subplans appear during a period of perhaps 8 million years (my) within the Early Cambrian, an unequaled explosion of morphological novelty, the ancestral lineages represented chiefly or entirely by trace fossils. Evidence from the fossil record can be combined with that from molecular phylogenetic trees to suggest that the last common ancestor of (i) protostomes and deuterostomes was a roundish worm with a blood vascular system and (ii) of arthropods and



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