The first photosynthetic oxygen-producing organisms on Earth were cyanobacteria. Their evolution dramatically changed the Earth allowing oxygen to accumulate into the atmosphere for the first time and further allowing the evolution of oxygen-utilizing organisms including eukaryotes. Eukaryotes include animals, but also algae, a broad group of photosynthetic oxygen-producing organisms that now dominate photosynthesis in the modern oceans. When, however, did algae begin to occupy marine ecosystems and compete with cyanobacteria as important phototrophic organisms?
