The origin of animal multicellularity and cell differentiation

How animals evolved from their single-celled ancestors over 600 million years ago is poorly understood. Comparisons of genomes from animals and their closest relatives – choanoflagellates, filastereans and ichthyosporeans – have recently revealed the genomic landscape of animal origins. However, the cell and developmental biology of the first animals have been less well examined. Using principles from evolutionary cell biology, we reason that the last common ancestor of animals and choanoflagellates (the ‘Urchoanozoan’) used a collar complex - a flagellum surrounded by a microvillar collar – to capture bacterial prey. The origin of animal multicellularity likely occurred through the modification of pre-existing mechanisms for extracellular matrix synthesis and regulation of cytokinesis. The progenitors of animals likely developed clonally through serial division of flagellated cells, giving rise to sheets of cells that folded into spheres by a morphogenetic process comparable to that seen in modern choanoflagellate rosettes and calcareous sponge embryos. Finally, we infer that cell differentiation evolved in the animal stem-lineage by a combination of three mechanisms: division of labor from ancient plurifunctional cell types, conversion of temporally segregated phenotypes into spatially segregated cell types, and functional innovation.