From Charles Q. Choi at Inside Science:
However, much remains unknown about when and how cyanobacteria evolved oxygenic photosynthesis. “The whole question of the origin of cyanobacteria has long been a mystery because they kind of just appeared out of the tree of life with this very advanced capability to do oxygenic photosynthesis without any apparent forebears,” said biochemist Robert Blankenship at Washington University in St. Louis, Missouri.
Until recently, all known cyanobacteria were photosynthetic members of class Oxyphotobacteria.
But in 2013, researchers discovered a nonphotosynthetic class of cyanobacteria known as Melainabacteria. Now Fischer and his colleagues have discovered a second class of nonphotosynthetic cyanobacteria, the Sericytochromatia. The researchers suggest that both groups are clearly closely related to photosynthetic cyanobacteria, based on their genomes, but the two groups do not perform photosynthesis themselves.
One possible explanation for the lack of photosynthesis in these two classes of cyanobacteria was that they could once photosynthesize but then lost the ability. More.
The plot deepens.
See also: Can we pinpoint the origin of oxygen photosynthesis?