Evolution of neuronal types and families.

Major questions in the evolution of neurons and nervous systems remain unsolved, such as the origin of the first neuron, the possible convergent evolution of neuronal phenotypes, and the transition from a relatively simple decentralized nerve net to the complex, centralized nervous systems found in modern bilaterian animals. In recent years, comparative single-cell transcriptomics has opened up new research avenues addressing these issues. Here, we review recent conceptual progress toward an evolutionary definition of cell types, and how it facilitates the identification and large-scale comparison of neuronal types and neuron type families from single-cell data - with the family of GABAergic neurons in distinct parts of the vertebrate forebrain as prime example. We also highlight strategies to infer cell type-specific innovation, so-called apomeres, from single-cell data.