The origin of multicellular animals from a unicellular protozoan represents a pivotal transition in life’s history. We are using choanoflagellate protozoans (close relatives of animals) that can be unicellular and can form multicellular colonies as a model system to study functional consequences of being colonial vs. unicellular. Our goal is to gain insights about selective factors that might have affected the evolution of multicellularity in the ancestors of animals. Each choanoflagellate cell propels water by beating a single flagellum and captures bacterial prey on a collar of microvilli around the flagellum. We have used high-speed microvideography and mathematical modeling to study the water flow produced by solitary cells and by colonies, and how that affects their performance of activities important to their survival and reproduction: feeding and avoiding predation.