Summary Little is known about how the first animals evolved from their single-celled ancestors. Over 120 years ago, Ernst Haeckel proposed that animals evolved through " repeated self-division of [a] primary cell, " [1] an idea supported by the observation that all animals develop from a single cell (the zygote) through successive rounds of cell division [2]. Nonetheless, there are multiple alternative hypotheses [3], including the formal possibility that multicellularity in the progenitor of animals occurred through cell aggregation, with embryogenesis by cell division being secondarily derived. The closest known relatives of animals, choanoflagellates, are emerging as a model system for testing specific hypotheses about animal origins [4–6]. Studying colony formation in choanoflagellates may provide a context for reconstructing the evolution of animal multicellularity. Here, we find that the transition from single cells to multicellular colonies in the choanoflagellate Salpingoeca rosetta (previously known as Proterospongia sp.) occurs by cell division, with sister cells remaining stably attached.