Abstract: Over 600 million years ago, the first multicellular metazoans evolved from their single celled ancestors. Although not recorded in the fossil record, the earliest events in metazoan evolution can be inferred by integrating findings from phylogenetics, genomics, and cell biology. Comparisons of choanoflagellate — microeukaryote relatives of metazoans, with sponges (the earliest known metazoans) — reveal genetic innovations associated with metazoan origins. Among these are the evolution of gene families required for cell adhesion and cell signaling, the presence of which catalyzed the evolution of multicellularity and whose functions have since been elaborated to regulate cell differentiation, developmental patterning, morphogenesis, and the functional integration of tissues. The most ancient tissues — differentiated epithelia — are found in sponges and evolved before the origin and diversification of modern phyla.

Keywords: early metazoan ancestors, choanoflagellates, sponges, origins of multicellularity,