The origin of animals from unicellular ancestors has been a key step in our evolutionary origins. However, the cellular and molecular basis of this transition remain incompletely understood.  In the past few years, key insights have emerged from the study of choanoflagellates: the closest living relatives of animals. I will summarize some of our recent research showing how choanoflagellate biology informs our understanding of several pivotal features of animal complexity, including multicellular development, morphogenesis, cell differentiation, and cell-cell communication.