Sponges are one of the simplest multicellular animals and are traditionally viewed as the oldest surviving animal clade. Similarities between choanocytes (the defining cell type for sponges) and choanoflagellates (single-cell and colonial protists) have long suggested an evolutionary link between them. This notion is supported by contemporary phylogenies which universally recover choanoflagellates as the sister group of animals, and in the majority of cases also place sponges as the earliest evolved animal lineage. Choanocytes combine functions which in many other animals are segregated between somatic cells and germ cells: in addition to capturing of food particles they are a source of gametes. These characteristics of choanocytes, combined with the phylogenetic position of sponges, suggest that sponge body plan might be a link between simple colonial protists and the complex body plans of “true animals”. My research program addresses this hypothesis by investigation of genes and gene regulatory networks governing specification of sponge body plan and cell types. An unexpected diversity and complexity of developmental regulatory gene repertoires among sponge species has been revealed as a fascinating (if somewhat frustrating) by-product of these studies.