Evolution of Multicellularity

Abstract

Abstract Multicellularity has evolved multiple times, independently, in lineages from all three domains of life. The transition from a unicellular to a multicellular life‐style entails the integration of previously independent individuals into a new kind of individual. The emergence of individuality at the higher level requires the evolution of cooperative and altruistic behaviours, division of labour (including the separation of reproductive and somatic activities among cells), the reorganisation of basic life properties (such as immortality and totipotency), and the reorganisation of fitness (with cells specialising in one or the other fitness components – survival or reproduction). At a more mechanistic level, a new genotype–phenotype map has to be created to reflect the emergence of a new kind of individual. Notably, many traits associated with multicellularity appear to have involved the co‐option of genes and pathways already present in unicellular lineages. The evolution of multicellularity has been driven by a combination of selective pressures, developmental constraints and life history trade‐offs specific to each lineage. Key Concepts: The transition from a unicellular to a multicellular life‐style constitutes a transition in individuality, a process whereby a group of previously independent individuals become stably integrated into a new functional, physiological and reproductively autonomous and indivisible evolutionary unit – that is, a new individual. The successful integration of previously independent individuals into a new kind of individual requires the evolution of cooperative and altruistic behaviours, division of labour, the reorganisation of basic life properties, and the reorganisation of fitness. Multicellularity has evolved independently in at least 25 lineages from all three domains of life. The evolution of multicellularity has been driven by a combination of selective pressures, developmental constraints and life history trade‐offs specific to each lineage. Many traits associated with multicellularity involved the co‐option of genes and pathways already present in unicellular lineages.