ABSTRACT
A critical step in the transition to multicellularity from a unicellular common ancestor was the origin of specialized cell types. The most recent unicellular ancestor of metazoans already had a complex gene complement, so increased genomic regulation must have played a vital role in the emergence of animal cell types and hence in the evolution of animals. Early branching animal groups like placozoans and sponges already have a range of differentiated cell types, but also, unicellular holozoans, the group formed by choanoflagellates, filastereans, and teretosporeans, often have complex life cycles that include multicellular stages and are characterized by spatially and temporally distinct cell morphologies with presumably distinct functions. Here, we review studies that illustrate how the complex life cycles of many of these groups involve diverse temporal cell states of varying morphology and physiology, governed by tight genetic regulation. The research reviewed provides compelling support for the idea that the raw material for the evolution of cell type-specific differentiation may have been already present in the unicellular ancestor of Metazoa. The use of single-cell RNA-seq data to characterize the cellular heterogeneity within the multicellular life stages of unicellular holozoans will provide important clues as to how cellular differentiation began in the closest unicellular relatives to animals.
