Conflict reducing innovations in development enable increased multicellular complexity

Obligately multicellular organisms, where cells can only reproduce as part of the group, have evolved multiple times across the tree of life. Obligate multicellularity has only evolved when clonal groups form by cell division, rather than by cells aggregating, as clonality prevents internal conflict. Yet obligately multicellular organisms still vary greatly in 'multicellular complexity' (the number of cells and cell types): some comprise few cells and cell-types, and others billions of cells and thousands of types. Here, we test if variation in multicellular complexity is explained by conflict-suppressing mechanisms, namely a single cell bottleneck at the start of development, and a strict separation of germline and somatic cells. Applying a phylogenetic comparative analysis to the life-cycles 138 lineages of plants, animals, fungi and algae, we show that an early segregation of the germline stem-cell lineage is key to the evolution of more cell types. In contrast, the presence of a strict single cell bottleneck was not related to either the number of cells or cell types but was associated with early germline segregation. Our results suggest that segregating the germline earlier in development enabled greater evolutionary innovation, possibly through conflict suppression or via greater developmental flexibility.