The Ediacaran and Cryogenian occurrences of 24-ipc are probably derived from stem-group demosponges (pink lines); these may or may not represent the true sponges, which exhibit multicellularity and a water-canal system. Indeed, there is no reason to rule out stem-group choanoflagellates now that these unicellular organisms are not considered directly ancestral to sponges13. a,b, 24-ipc is limited to true sponges. c,d, These schemes allow the possibility of (now-extinct) pre-sponge, non-metazoan organisms as the source; the phylogeny in (d) is currently the most strongly supported account of poriferan relationships at the class level9. The most recent molecular phylogeny10 suggests that the most primitive animals are not sponges but ctenophores, a group of animals that superficially resemble cnidarian jellyfish, but that belong to a separate phylum (green dotted line in d). Choano, Choanoflagellida; Dem, Demospongiae; Hex, Hexactinellida; Calc, Calcarea; Eum, Eumetazoa; Ctn, Ctenophora.