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Abstract:
Cooperation is central to the emergence ofmulticellular life; however, the means bywhich the earliest collectives (groups
of cells) maintained integrity in the face of destructive cheating types is unclear. One idea posits cheats as a primitive germ
line in a life cycle that facilitates collective reproduction. Here we describe an experiment in which simple cooperating
lineages of bacteria were propagated under a selective regime that rewarded collective-level persistence. Collectives
reproduced via life cycles that either embraced, or purged, cheating types. When embraced, the life cycle alternated
between phenotypic states. Selection fostered inception of a developmental switch that underpinned the emergence of
collectives whose fitness, during the course of evolution, became decoupled from the fitness of constituent cells. Such
development and decoupling did not occur when groups reproduced via a cheat-purging regime. Our findings capture
key events in the evolution of Darwinian individuality during the transition from single cells to multicellularity.
