A model for the evolution of the bHLH complement in metazoans. A simplified phylogenetic tree of the opisthokonts is represented. The three main groups of opisthokonts – the fungi, the choanoflagellates and the metazoans – are indicated. For the metazoans, we have represented their main subdivisions: the two probable monophyletic groups of 'sponges', the demosponges + hexactinellids, and the calcareous sponges; the cnidarians; and the two main groups of bilaterians, the protostomes and the deuterostomes. Last common ancestors of opisthokonts (Uropisthokonta), metazoans (Urmetazoa), eumetazoans (Ureumetazoa), and bilaterians (Urbilateria) are represented by black polygons. The minimal number of bHLHs inferred to have been present in these ancestors is shown (light grey boxes). We also indicate the minimal inferred number of families for the different higher-order groups present in each last common ancestor.