Perspective—Evolution of Neural Cell Types

Abstract The nervous system is composed of cell types. Thus, any attempt to elucidate the evolutionary origin of the nervous system requires comparison of the constituting neural cell types. This chapter surveys the state of the art in the comparative analysis of cell types in the nervous system, conceptually and by means of a few illustrative examples. First, what is a cell type? In essence, the definition of a ‘cell type’ is structural. It refers to the specific phenotype of a population of cells in the body that are structurally more similar to each other than to other cells. Therefore, neural cell types are defined by their specific structure reflecting their unique function. The structure of a given cell type is a manifestation of its molecular composition; and although cell types are more than the sum of their constituent parts, the comparative analysis of their molecular composition in various different ways is key to understanding cell type evolution. Typically, cellular functions require the cooperation of proteins and other biomolecules that constitute ‘modules’. We can thus envisage a neural cell type, as any other cell type, as an assembly of modules with discrete subfunctions. The focus of this chapter is on the evolutionary origin of ‘neural’ modules in early metazoans and their subsequent distribution to emergent sister cell types such as sensory neurons and interneurons.