Ruth
A. DewelAdjunct Professor and
Director of the Electron Microscopy Laboratory
Ph.D., University of Houston
Our understanding of the early evolution of metazoans has been limited by the popular assumption that early forms were either larva- or flatworm-like. This presumption, however, is being challenged by data from morphology, paleontology, and more recently molecular developmental biology that suggest that the first animals to evolve were unexpectedly complex. For example, there is growing evidence that the last common ancestor of bilaterally symmetrical triploblasts had a head, segmented trunk, and a tail, and possessed a dorsal brain and photoreceptors. "Appendages" or outgrowths extending perpendicular to the main body axis were also present. The trunk contained serially repeated gonads and gill slits and pores, and a dorsal heart pumpedblood through vessels or connective tissue spaces between coelomic cavities. Although earlier theories propose that bilaterians arose from a tiny, simple, and cryptic ancestor, an alternative hypothesis based on a macroscopic ancestor is consistent with the available evidence. This hypothesis suggests that complex bilaterians arose through individuation of a colony of cnidarian-grade organisms and that an entire colony of organisms, perhaps similar to modern pennatulacean anthozoans, gave rise to the Bilateria. Such a scenario fits the data surprisingly well, and if correct, holds important implications for understanding the origin of more primitive metazoans. The hypothesis also may help unravel the mystery of the Ediacarans, an important but enigmatic component of the Precambrian fossil record. These organisms appear exceedingly primitive and yet exhibit characters such as bilateral symmetry, segmentation, and a "head" and "trunk," which are considered to typify derived animals. According to the "colony theory," these features would have arisen through duplication and specialization of simple colonies of cells before or shortly after the origin of sponges. Thus morphological complexity originated early in the evolution of Metazoa, and macroscopic and modularized organisms may have characterized the fauna populating the Vendian oceans long before the appearance of bilaterians or cnidarians.
