Meeting Abstract
Early evolution of animals has long been a mystery. The fossil record of the earliest metazoan forms is very limited and there has been disagreement between various molecular phylogenetic analyses as to which extant lineage is sister to all other animals. Phylogenomic evidence suggests that ctenophores branched off from other animals earlier than sponges. Here we will review how data support and conflict with alternative hypotheses about the base of the animal tree. Strengths and pitfalls of recent studies will be examined with attention paid to the sponge-first versus ctenophore-first topologies. These alternative hypotheses imply different scenarios about the evolution of tissue types and organ systems. In particular, muscular and neural systems vary greatly among basal animal lineages unlike bilaterian animals where construction and organization of these systems seem to follow set rules. Importantly, the long held assumption that sponges are basal animals has constrained which features and characters we have focused on as evolutionarily important. In order to understand evolutionary history of morphological features, we must explore complex pathways and gene systems that allow features, such as nerves and muscles, to function in an integrated capacity. Placement of ctenophores at the base of the animal tree implies that metazoan nervous systems evolved twice, moreover functional and genomic evidence shows that ctenophore neural systems are very unlike those of other animals regardless of inferred tree topology. Discussion will focus on the interplay between inferred tree topologies and interpretation of morphological features (muscles and neural systems) in early animals.
