Abstract

Background: Cyanobacteria are one of the oldest and morphologically most diverse prokaryotic phyla on our
planet. The early development of an oxygen-containing atmosphere approximately 2.45 - 2.22 billion years ago is
attributed to the photosynthetic activity of cyanobacteria. Furthermore, they are one of the few prokaryotic
phyla where multicellularity has evolved. Understanding when and how multicellularity evolved in these ancient
organisms would provide fundamental information on the early history of life and further our knowledge of
complex life forms.
Results: We conducted and compared phylogenetic analyses of 16S rDNA sequences from a large sample of taxa
representing the morphological and genetic diversity of cyanobacteria. We reconstructed ancestral character
states on 10,000 phylogenetic trees. The results suggest that the majority of extant cyanobacteria descend from
multicellular ancestors. Reversals to unicellularity occurred at least 5 times. Multicellularity was established
again at least once within a single-celled clade. Comparison to the fossil record supports an early origin of
multicellularity, possibly as early as the “Great Oxygenation Event” that occurred 2.45 - 2.22 billion years ago.
Conclusions: The results indicate that a multicellular morphotype evolved early in the cyanobacterial lineage and
was regained at least once after a previous loss. Most of the morphological diversity exhibited in cyanobacteria
today —including the majority of single-celled species— arose from ancient multicellular lineages.
Multicellularity could have conferred a considerable advantage for exploring new niches and hence facilitated the
diversification of new lineages.