A Look Back Over 20 Years of Evo-Devo Studies on Sponges: A Challenged View of Urmetazoa

The majority of extant metazoans are animals with bilateral symmetry, a gut, a central nervous system and a head; they are grouped in a taxon named Bilateria. As a consequence, most of the zoological definitions are based on observations in bilaterian animals. To understand how these anatomical features emerged, it is now necessary for the Evo-Devo field to undertake studies on the four extant lineages that emerged earlier, namely Cnidaria, Placozoa, Ctenophora and Porifera. Being the sister group of bilaterians, cnidarians have received much attention compared to the other three phyla but, given the disparity of body plans between these four non-bilaterian phyla, comparative analyses are needed not only to trace back the origin and evolution of genetic mechanisms involved in animal development but also to understand the evolutionary processes that gave rise to such a huge animal body plan diversity. This chapter focuses on the evo-devo approaches applied to Porifera and how the data have changed our view of these uncommon animals and how it challenges previous concepts of the last common metazoan ancestor. We discuss here the current and future steps that need to be undertaken to ensure that sponges join the laboratory "model organisms club."