Eukaryotic photoautotrophs include red algae, haptophytes, stramenopiles, cryptophytes, chlorophytes, and land plants. These organisms perform photosynthesis through organelles called chloroplasts and are believed to have originated about 2 billion years ago. Comparing the genes of chloroplast and cyanobacteria strongly suggests that chloroplasts evolved as a result of endosymbiosis with cyanobacteria that gradually lost the genes required to be free-living. However, it is difficult to determine whether all chloroplasts originated from a single, primary endosymbiotic event, or multiple independent events. Some brachiopods (Gigantoproductus) and bivalves (Tridacna) also evolved photoautotrophy.