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Abstract The evolution of multicellularity led to the origin of new kinds of organisms and, in several lineages, massive adaptive radiations through the formation of entirely new ecosystems. This paper examines three key mechanisms underpinning parallel adaptive radiations within the five clades of ‘complex’ multicellularity: animals, land plants, fungi, red algae, and brown algae. First, the evolution of key multicellular innovations permitted diversification into new ecological roles. Second, the evolution of large multicellular organisms with strong genetic bottlenecks between generations fundamentally changed the population genetic context of evolution, greatly reducing effective population size and increasing the role of genetic drift. This may be beneficial during adaptive radiations, underpinning nonadaptive expansions of genome size and allowing broader exploration of multicellular trait space. Finally, we explore how evolutionary priority effects provide a first-mover advantage, maintaining ancient adaptive radiations over long time periods by suppressing competition from convergently evolving multicellular taxa. Investigating parallel patterns of diversification across independent origins of complex multicellularity provides insight into the principles underpinning these crucially important adaptive radiations.