A close unicellular relative reveals aggregative multicellularity was key to the evolution of animals.

How animals evolved complex multicellularity from their unicellular ancestors remains unanswered. Unicellular relatives of animals exhibit simple multicellularity through clonal division, formation of multinucleate coenocytes, or aggregation. Therefore, animal multicellularity may have evolved from one (or a combination) of these behaviours. Aggregation has classically been dismissed as a means to complex multicellularity. However, aggregation occurs in many extant animal cells and has also been recently described in three different unicellular relatives of animals (the choanoflagellates and , and the filasterean ). It is unclear whether aggregation in these species is derived or ancestral, and its relevance for animal origins remains unknown. To fill this gap, we investigated whether an additional unicellular relative of animals can undergo aggregation. We discovered that the marine free-living bacterivorous filasterean forms homogeneous aggregates with reproducible kinetics that have long-term stability when cultured with an alphaproteobacterium. We found that many multicellularity genes involved in animal cell adhesion, signaling, and transcriptional regulation were deployed during this process. Our findings suggest that the last unicellular ancestor of animals had the capacity to aggregate using key animal multicellularity genes and that improved feeding and sexual reproduction may be evolutionary drivers of this aggregation.