Caroline Rose, Katrin Hammerschmidt

The meaning of the word ‘multicellularity’ appears to be unambiguous – a concept that can be grasped with common sense. On closer inspection, however, there is notable disparity in the recent literature regarding the usage of the term ‘multicellularity’, which can describe complex organisms, simple microbial colonies or even multi-species biofilms. In addition, while emerging research directions, such as ‘the evolutionary transition to multicellularity’, have brought together a highly interdisciplinary community of researchers, they tend to (mis)use historically relevant language that does not adequately describe the marginal or nascent cases of multicellularity that are the focus of this new field. We argue that clarity can be achieved with the realisation that the various definitions of multicellularity are in fact describing different stages that can occur during the course of evolution. With the aim of gaining semantic continuity among researchers in this broad field, we provide a framework for identifying the stages of the evolutionary transition to multicellularity: from multicellular groups, to multicellular individuals, and finally to multicellular organisms.