On the left, organisms that evolved multicellularity by aggregative development have a life cycle where individuals from the environment aggregate, and cooperate to form a multicellular organism. These cells need not be genetically identical (indicated by the different colors). In many organisms with this life cycle, only some cells are dispersed for reproduction. On the right, organisms that evolved multicellularity by clonal development remain attached together after each cell division, forming groups of undifferentiated cells. Each cell in the group can produce a genetically identical reproductive cell, or ‘propagule’, that produces genetically uniform offspring. If aggregative development and clonal development are both important for multicellular evolution in metazoa, as the results of Sebé-Pedrós et al. suggest, then a new unified mechanism—‘cooperative clonal development’—is required.