Seminal work of Raper showed similarity of principle in the establishment of the body plan in Dictyostelium and vertebrate development. In vertebrate development, a small group of cells known as ''the organizer'' releases signals that coordinate cell movement during gastrulation and neurulation and thereby generate the animal's head-to-tail body axis. (4) Raper demonstrated that, in Dictyostelium aggregates, small groups of cells, recognizable as tips, secretes signals that generate anteroposterior polarity of all or a subpopulation of cells in the aggregate, yielding one or several slugs with a distinct head and body region. (5) More recent work shows that animals and Dictyostelia share many conserved pathways for processing external signals. Particularly the elucidation of the processes that control chemotaxis in Dictyostelium have become a paradigm for understanding cell migration in animals. (6) (7) (8) However, the external signals that trigger movement or differentiation are rarely conserved. For instance, growth-factor-like peptides and their tyrosine kinase receptors that play such crucial roles in animal development are not present in Dictyostelium. The homeobox-containing transcription factors that specify segment identity in arthropods and vertebrates have only a minor function in Dictyostelium development. (9) (10) (11) The signaling repertoire of the Dictyostelia is rather different. The model species, D. discoideum makes extensive use of cyclic AMP (cAMP), the ubiquitous intracellular messenger for hormone action in vertebrates. In D. discoideum, cAMP not only mediates the effect of a number of external signals, but is also secreted to act as a chemoattractant and inducer of cell differentiation. (12) Secreted peptides trigger maturation of spores, but are detected by sensor-coupled histidine kinases. (13) Polyketide-based metabolites and adenine-based cytokinins are secreted to regulate cell-type proportioning and spore germination respectively. (14, 15) Animals, plants and Dictyostelia evolved from different unicellular ancestors, supposedly the filter-feeding choanoflagellates for animals, (16) green algae for plants (17) and solitary amoebas for the Dictyostelia. (18) These unicellular progenitors used sensory signaling to monitor their environment and to find food and mates. The different developmental strategies that are now used by their multicellular descendants reflect how evolutionary forces differentially selected, duplicated and adapted these environmental sensing mechanisms for increasingly complex communication between cells.