New perspectives on the origin of metazoan complexity: an introduction to the symposium.

Impressive gains have been made in our understanding of metazoan complexity at the molecular level, especially those in the field of developmental evolution, but our understanding at other levels has lagged behind. This symposium attempts to bring diverse perspectives to bear on the causes and mechanisms of the explosive growth in metazoan complexity in the late Precambrian and Lower Cambrian. A primary goal is to broaden the framework for study of animal origins by introducing topics that are often overlooked by comparative biologists. An additional goal is to bring together workers that hold very different views on the early evolution of metazoans. For example, paleontologists and neontologists vary widely in their interpretations of a remarkable assemblage of fossils, the socalled Ediacara biota that existed in the latest Neoproterozoic (600–543 Ma). This assemblage is widely regarded as belonging to a metazoan or premetazoan radiation by paleontologists and yet the construction of these organisms is rarely taken into consideration by neontologists. Very often metazoan ancestors are assumed to have been a simple larva or flatworm-like organism, such as the microscopic acoelomate considered to have been the bilaterian ancestor (see Rieger et al., 1991) in spite of the fact that many Ediacarans are large frondor slug-like organisms, and highly modular. The theory that early metazoans were simple and that complexity has been assembled incrementally is flawed for other reasons. Such a trajectory is not required by logic, because the first metazoans could have been complex, with complexity decreasing in some or most clades. In addition, a ‘‘complexity late’’ scenario provides no clues about the structure of the intermediate forms connecting the simple common ancestor with later more complex forms; indeed, little work of even a speculative nature has been done to address this issue (Conway Morris, 2000). The assumption is also challenged by the discovery that protostomes and deuterostomes share complex developmental programs, such as the Hox cluster regulating spatial patterning (Balavoine and Adoutte, 1998; de Rosa et al., 1999; Holland, 1998), and that even cnidarians possess a complex set of Hox genes (Finnerty and Martindale, 1999; Gauchat et al., 2000; Kourakis and Martindale, 2000). Although genetic complexity usually is thought to have preceded increases in body plan complexity,