What did the evolution of multicellular animals look like?
The life cycle of myxobacteria (e.g., Myxococcus coralloides, Myxococcus disciformis, Myxococcus flavescens, Myxococcus fulvus, Myxococcus macrosporus, Myxococcus stipitatus, Myxococcus virescens, Myxococcus xanthus---for references to the original articles describing these species, see Bacteria I. Taxonomy: Genera and Species) includes the formation of fruiting bodies, which is an obligate multicellular process requiring cooperativity between individual bacterial cells (see, e.g., Cao et al., 2015).