The Biphasic Life Cycle—A Central Theme of Metazoan Evolution

Abstract

SYNOPSIS. A life cycle alternating between a microscopic larva (mmsized) and macroscopic adult (cm-sized) occurs commonly among invertebrates. The pattern of occurrence of such a biphasic life cycle among lower metazoans like the Porifera and Cnidaria, as well as ultrastructure of larva and adults of many invertebrates, suggests that the earliest phases of metazoan evolution also had such a cycle. Only the dispersive larvae in the original metazoans could be characterized as “small and mobile” if we follow this model to its logical conclusion; the adults, by contrast, evolved into larger-bodied organisms to maximize reproduction and were non-motile filter feeders for the most part. The biphasic life cycle arose early in evolution, then, shortly after the first multicellular organisms evolved as clones of flagellated cells linked together by interactions through ECM; selective pressures acting independently on the two phases of the life cycle led to the origin of the different metazoan tissue types—epithelial, connective, nervous, and muscle tissues. For the Bilateria, this suggests that extant acoelomate and pseudocoelomate forms evolved through progenesis of larvae and juveniles in a manner like that known for the radiation of the interstitial annelids. A better understanding of larval biology and of mechanisms for heterochrony is crucial for hypotheses of metazoan evolution.