Evolution of Multicellularity
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International audience. The emergence of multicellular organisms was perhaps the most spectacular of the major transitions during the evolutionary history of life on this planet [1,2]. Despite decades of research aimed at understanding this important transition, many questions remain unanswered. This special issue, "Evolution of Multicellularity", which consists of reviews, an opinion piece and original research articles, highlights and addresses some of these questions. Together these articles look at several aspects of the evolution of multicellularity, ranging from the evolution of key molecular components to the emergence of cellular and organism-level features. They consider both early events at the transition between unicellularity and multicellularity and the subsequent complexification of multicellular organisms. Together the articles present several novel observations and propose interesting new hypotheses aimed at stimulating future research in this area. Research into the evolution of multicellularity tends to involve either studies that try to understand transitions from unicellularity to multicellularity or studies that focus on the subsequent step, corresponding to the emergence of complex multicellularity (i.e. the evolution of macroscopic organisms with multiple cell types such as animals and land plants). Kin and Schaap [3] argue that this distinction is somewhat artificial and point out that, in fact, lineages may exhibit a continuum of different levels of complexity and that this continuum can provide useful material for investigating the evolutionary origins of multicellularity. One such lineage is the dictyostelid social amoebas. Analysis of this lineage supports a broad hypothesis that is probably relevant to diverse multicellular lineages [4-7]: that the adaptation of what the authors call "proto cell-types" to particular environmental conditions may have laid the foundations for the later emergence of distinct cell types in a multicellular decedent.
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