Life Cycles as a Central Organizing Theme for Studying Multicellularity

Multicellular organisms exhibit a wide diversity in multicellular life cycles that differ in how development and reproduction are achieved. Every multicellular life cycle comprises group formation (via staying together or coming together, i.e., clonal or aggregative multicellularity) and group propagation. Many of these life cycles contain facultative life stages that are only expressed under specific environmental conditions. While a life cycle-centered approach is crucial to understanding the evolution of multicellularity, many open questions remain. In order to answer these questions, a bottom-up approach to the evolution of multicellularity-which aims to characterize evolutionary trajectories towards multicellularity as they unfold from a specific unicellular starting point-can be a useful complement to the traditional top-down approach. As an explicit example, we discuss a recent theoretical model that shows that diverse multicellular life cycles can readily emerge from a unicellular ancestor, shaped by ancestral constraints and environmental conditions. We envision that future progress will depend on sustained dialog between the bottom-up and top-down perspectives.