Neurons are highly specialized cells present in nearly all animals, but their evolutionary origin and relationship to other cell types are not well understood. We use here the sea anemone as a model system for early-branching animals to gain fresh insights into the evolutionary history of neurons. We generated a transgenic reporter line to show that the transcription factor is expressed in postmitotic cells that give rise to various types of neurons and secretory cells. Expression analyses, double transgenics, and gene knockdown experiments show that the -expressing neurons and secretory cells derive from a common pool of -positive progenitor cells. These findings, together with the requirement for for the development of neurons and endocrine cells in vertebrates, support a close evolutionary relationship of neurons and secretory cells.