THE BIPHASIC LIFE-CYCLE - A CENTRAL THEME OF METAZOAN EVOLUTION

A life cycle alternating between a microscopic larva (mm-sized) and ma croscopic adult (cm-sized) occurs commonly among invertebrates. The pa ttern of occurrence of such a biphasic life cycle among lower metazoan s like the Porifera and Cnidaria, as well as ultrastructure of larva a nd adults of many invertebrates, suggests that the earliest phases of metazoan evolution also had such a cycle. Only the dispersive larvae i n the original metazoans could be characterized as ''small and mobile' ' if we follow this model to its logical conclusion; the adults, by co ntrast, evolved into larger-bodied organisms to maximize reproduction and were non-motile filter feeders for the most part. The biphasic lif e cycle arose early in evolution, then, shortly after the first multic ellular organisms evolved as clones of flagellated cells linked togeth er by interactions through ECM; selective pressures acting independent ly on the two phases of the Life cycle led to the origin of the differ ent metazoan tissue types-epithelial, connective, nervous, and muscle tissues. For the Bilateria, this suggests that extant acoelomate and p seudocoelomate forms evolved through progenesis of larvae and juvenile s in a manner like that known for the radiation of the interstitial an nelids. A better understanding of larval biology and of mechanisms for heterochrony is crucial for hypotheses of metazoan evolution.