Cyanobacteria, considered responsible for the Great Oxidation Event (GOE) that shaped the Earth for the evolution of complex life, are among the most morphologically diverse prokaryotic phyla. Their morphotypes range from unicellular to multicellular filaments and, additionally, many filamentous cyanobacteria exhibit cellular differentiation processes. However, mechanisms underlying the evolution of filamentous morphologies remain unknown. Here, we implement phylogenomic, Bayesian molecular clock and gene-tree-species-tree reconciliation analyses to estimate when genes encoding cell-cell joining structures—involved in multicellularity—and cellular differentiation regulators first evolved. Our results suggest that genes encoding septal proteins (namely sepJ and sepI ) and proteins potentially involved in the formation of patterns of distinct cells ( hetR ) evolved in the Neoarchaean ~2.6-2.7 billion years ago (Ga). Later, at the start of the GOE ~2.5 Ga, genes involved in cellular differentiation (namely hetZ, patU3 ) and increased septal complexity ( hglK ) appeared. We also characterise septal structures and measure intercellular molecular transfer in non-model and early-branching filamentous strains, finding intercellular molecular exchange in phylogenetically distant filamentous cyanobacteria. Our results predict that early-branching lineages like Pseudanabaena were capable of intercellular exchange in the lead up to the GOE, despite having fewer septal pores than more-recently-evolving heterocystous strains.
