Multispecies interactions shape the transition to multicellularity.

The origin of multicellularity transformed the adaptive landscape on Earth, opening diverse avenues for further innovation. The transition to multicellular life is understood as the evolution of cooperative groups which form a new level of individuality. Despite the potential for community-level interactions, most studies have not addressed the competitive context of this transition, such as competition between species. Here, we explore how interspecific competition shapes the emergence of multicellularity in an experimental system with two yeast species, Saccharomyces cerevisiae and Kluyveromyces lactis , where multicellularity evolves in response to selection for faster settling ability. We find that the multispecies context slows the rate of the transition to multicellularity, and the transition to multicellularity significantly impacts community composition. Multicellular K. lactis emerges first and sweeps through populations in monocultures faster than in cocultures with S. cerevisiae . Following the transition, the between-species competitive dynamics shift, likely in part to intraspecific cooperation in K. lactis . Hence, we document an eco-evolutionary feedback across the transition to multicellularity, underscoring how ecological context is critical for understanding the causes and consequences of innovation. By including two species, we demonstrate that cooperation and competition across several biological scales shapes the origin and persistence of multicellularity.