The volvocine green algae are a model system for the evolution of multicellularity and cellular differentiation. A combination of molecular genetic and phylogenetic comparative approaches has resulted in a detailed picture of the transition from single cells to differentiated, multicellular organisms in this group. To be useful as a model system, the volvocine algae should provide information that is relevant to other groups. Here I discuss recent advances in understanding the origins of multicellularity and cellular differentiation in the volvocine algae and consider the implications for such transitions in general. Several general principles emerge that are relevant to the origins of major multicellular groups, such as animals, plants, fungi, red and brown algae. First, if the lessons learned from the volvocine algae can be generalized to other origins of multicellularity, we should expect these transitions to be understandable as a series of small changes, each potentially adaptive in itself. In addition, cooperation, conflict and mediation of conflicts among cells are likely to have played central roles. Finally, we should expect the histories of these transitions to include parallel evolution of some traits, periods of relatively rapid change interspersed with long periods of stasis, and simpler forms coexisting with more complex forms for long periods of time as in the evolution of the volvocine algae.
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