Answer:
stromatolites
Step-by-step explanation:
"stromatolites is a calcareous mound built up of layers of lime-secreting cyanobacteria and trapped sediment, found in Precambrian rocks as the earliest known fossils, and still being formed in lagoons in Australasia"
Ancient cyanobacteria present in stromatolites are considered as key factor for the increased amount of oxygen in atmosphere. Furthermore, these were considered as first organisms to trap carbon dioxide and release oxygen by the process of photosynthesis.