Cell types are the fundamental units of multicellular life, which have diversified during animal evolution. The ongoing revolution in single-cell genomics/transcriptomics technologies and new insights into the molecular mechanisms specifying cell type identity now allow us to explore this process in unprecedented detail. Our new EMBO | EMBL Symposium will bring together scientists in this emerging field. We will jointly discuss fundamental questions such as the origins of cell types in the evolution of multicellularity, their diversification in divergent animal lineages and the molecular evolution of regulatory networks underlying the specification of cell types and tissues. One focus will be on the evolution of neuron type identity, and the cellular origins of the vertebrate cortex. We will also explore new computational approaches to unravel whole-body single-cell gene regulatory networks that underlie cell type diversification. For the first time, our EMBO | EMBL Symposium will provide a forum for the emerging field of cell type and tissue origination in the single-cell genomics era.