Salpingoeca rosetta as a model for studying the ancestry of metazoan multicellularity. (a) Choanoflagellates are the closest living relatives of Metazoa [1, 3, 68]. Taxonomic groupings are indicated above the phylogeny and the last common ancestors of each group are indicated as colored circles at nodes. The topology of the reference phylogeny was based on a consensus of results from [68–70]. Branches were collapsed at the base of Metazoa to reflect current uncertainty about the identity and branch order of the most basal metazoan phyla [69, 71–73]. The black, yellow, and green colored nodes are used in both Figures 1 and 2 to represent the Urmetazoan, Urchoanimal, and Uropisthokont, respectively. (b) The evolution of metazoans from their single-celled ancestors is hypothesized to have involved a transition through a simple colonial form, such as Haeckel's Blastea (left, from Figure 117 of ) or Nielsen's Choanoblastea (center, from ), that resembles the rosette colonies formed by S. rosetta (right). (c) S. rosetta can transition through at least five morphologically and behaviorally differentiated cell types . Solitary 'thecate' cells attached to a substrate (Th) can produce solitary swimming (Sw) cells or solitary fast swimming (FS) cells, either through cell division or theca abandonment. Solitary swimming cells can divide completely to produce solitary daughter cells or remain attached after undergoing incomplete cytokinesis to produce either chain colonies (CC), or rosette colonies (RC) in the presence of the bacterium Algoriphagus machipongonensis (asterisk) [6, 18, 64]. Fil., Filasterea; Cho., Choanoflagellates.