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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. 

The diversity of multicellular organisms is, in large part, due to the fact that multicellularity has independently evolved many times. Nonetheless, multicellular organisms all share a universal biophysical trait: cells are attached to each other. All mechanisms of cellular attachment belong to one of two broad classes; intercellular bonds are either reformable or they are not. Both classes of multicellular assembly are common in nature, having independently evolved dozens of times. In this review, we detail these varied mechanisms as they exist in multicellular organisms. We also discuss the evolutionary implications of different intercellular attachment mechanisms on nascent multicellular organisms. The type of intercellular bond present during early steps in the transition to multicellularity constrains future evolutionary and biophysical dynamics for the lineage, affecting the origin of multicellular life cycles, cell–cell communication, cellular differentiation, and multicellular morphogenesis. The types of intercellular bonds used by multicellular organisms may thus result in some of the most impactful historical constraints on the evolution of multicellularity. 

Abstract The transition from unicellular to multicellular life was one of a few major events in the history of life that created new opportunities for more complex biological systems to evolve. Predation is hypothesized as one selective pressure that may have driven the evolution of multicellularity. Here we show thatde novoorigins of simple multicellularity can evolve in response to predation. We subjected outcrossed populations of the unicellular green algaChlamydomonas reinhardtiito selection by the filter-feeding predatorParamecium tetraurelia. Two of five experimental populations evolved multicellular structures not observed in unselected control populations within ~750 asexual generations. Considerable variation exists in the evolved multicellular life cycles, with both cell number and propagule size varying among isolates. Survival assays show that evolved multicellular traits provide effective protection against predation. These results support the hypothesis that selection imposed by predators may have played a role in some origins of multicellularity.