When did life in the oceans start?

Life in the oceans started around 3.5 billion years ago. This is based on evidence from stromatolites, which are layered rock structures that are formed by the growth of cyanobacteria. Cyanobacteria are photosynthetic bacteria that produce oxygen as a byproduct of photosynthesis. The presence of stromatolites in rocks from 3.5 billion years ago suggests that there were already cyanobacteria living in the oceans at that time.