Progress And Perspectives Of The Deep Non-Bilaterian Phylogeny, With Focus On Sponges (Phylum Porifera)

Sponges (Phylum Porifera) are benthic aquatic animals of great ecological and pharmaceutical importance. The phylum is arguably the earliest branching metazoan taxon and of considerable significance for the reconstruction of early metazoan evolution. However, the deep phylogeny of sponges is still debated, as the exact branching pattern of their main clades and their relationships to the other non-bilaterian animals is not unequivocally resolved. Here, the current state of the debate about the deep phylogeny of sponges and other non-bilaterians is reviewed and we highlight some issues that need to be considered when using phylogenomic data to reconstruct these relationships. Several studies, mainly based on single or a few genes, have suggested that sponges are a paraphyletic assemblage that share a grade of construction rather than common ancestry. However, recent phylogenomic analyses have suggested that sponges are monophyletic, consistent with earlier cladistic analyses based on morphological characters. This finding has many implications for the evolutionary interpretation of early animal traits. However, we conclude that further evidence from sequence-independent characters needs to be sought to test existing phylogenetic hypotheses. Only by cross-corroboration with independent data might we come closer to fully resolving the deepest branches of the animal tree of life.