Multicellular morphologies have evolved over 50 times in the history of life on Earth, across both pro- and eukaryotic lineages. Indeed, some prokaryotes have achieved versions of multicellularity—for example, some photosynthetic cyanobacteria partition labor into two cell types, where one cell type carries out photosynthesis while the other fixes nitrogen. Meanwhile, myxobacteria form multicellular fruiting bodies as a part of their complex life cycle.

Yet, despite that fact that some prokaryotes have skirted complex multicellularity with these multicellular lifestyles, no prokaryotic lineages have achieved complex multicellularity in the way that eukaryotes have—with several kinds of cells working together and organizing into a larger organism.

Indeed, complex multicellularity has only evolved five separate times, always within eukaryotes. This observation underlies a fundamental question in evolutionary biology: although prokaryotic cells have complex cellular structures and behaviors and have had plenty of time to evolve complex multicellularity, they have never done so—why not?