Photo courtesy Marjory Martin, Deakin Univ, Australia Very rarely, stromatolites are still forming today in places such as Shark Bay, Australia. The salinity of the water is very high in this bay, and basically the only life that can survive is cyanobacteria. Under normal ocean conditions, the cyanobacteria would be eaten by marine creatures such as snails. In this special case however, columns and mounds are forming as can be seen in the picture to the left. Being photosynthetic, cyanobacteria produce oxygen as a by-product.
Photosynthesis is the only major source of free oxygen gas in the atmosphere.
This major change paved the way for the next evolutionary step, the appearance of life based on the eukaryotic cell.