They were microbial mats. From Anna Azvolinsky at The Scientist:
According to the study authors, the previously oldest visible fossilized remains of microbes on land were about 2.7 billion years old, found in a different location from the Barberton Greenstone Belt in South Africa and also in Australia … For the current study, Homann and his colleagues focused on ancient sedimentary rocks, known as the Moodies Group, in the Barberton Greenstone Belt that were shown by geologists earlier to be approximately 3.22 billion years old. There, the team uncovered what are known as fossilized microbial mats—composed mainly of the imprints of bacteria and archaea and are among the earliest preserved forms of life. While living on the early Earth, these microbial community mats became interlayered and packed together with sedimentary rock made of rounded stones of different sizes that geologists call a conglomerate.
For Konhauser, it would be interesting to dig deeper into the source of the nitrate in the samples and whether it might have indeed come from the atmosphere or via generation of oxygen from the ancient photosynthetic bacteria. “The structures and isotopic composition of the microbial mats certainly seem to suggest the presence of photosynthetic microbes already existing on land,” writes Konhauser. If the nitrate were indeed formed by the microbes in the mats, he adds, then perhaps oxygen-producing cyanobacteria were around at this early stage of the Earth’s history. More.
At a certain point, that ol’ Darwinian magic could just lose the adjective and become “”that ol’ magic.” Alternatively, evolution did not happen the way naturalists (nature is all there is) claim.
See also: Soil micro-organisms older than thought