Chloroplasts are thought to have evolved from free-living photosynthetic cyanobacteria.
Photosynthesis is responsible for the conversion of Earth's early atmosphere to one that is oxygen-based. It is believed to have originated about 3.5 billion years ago in a type of bacteria known as cyanobacteria. Cyanobacteria joined already existing cells as an endosymbiont, which means these organisms lived inside the cells of other organisms in symbiosis, mutually benefiting one another. Thus, cyanobacteria were providing their host cells with sugars. While living inside the host cell, the cyanobacterium was not totally ingested. This left the newly formed chloroplast with two outer membranes. The existing host cell already had mitochondria, which it used to create energy for itself. The cyanobacterium brought the ability to use sunlight to split water molecules and drive the conversion of carbon dioxide into organic molecules. Over time, many of the genes from the cyanobacterium migrated into the genome of the host cell, completing the interdependence. This made the host cell completely dependent upon photosynthesis for energy. These host cells, and all photosynthetic plants, use this process to inherit chloroplasts. From an evolutionary standpoint, the formation of the symbiotic relationship between the cyanobacterium and the host cell happened only one time. Once merged, the host cell diverged into four distinct lineages of algae and algae-like organisms—the green algae, the red algae, the brown algae, and the glaucophytes. Green algae, such as sea lettuce (Ulva), are believed to be the precursors of modern land plants, giving rise to more than 400,000 species alive today. Red algae, such as Irish moss (Chondrus), produce a compound called carrageenan, which is used in the food industry as a thickening agent in products such as chocolate milk and ice cream. Brown algae, such as kelp (Fucus), are used in the sushi industry to wrap fish and other seafood. Glaucophytes are unicellular freshwater algae that are believed to resemble the original endosymbiont of the chloroplast.
Since the original evolution of the chloroplast, there have been several other events that led to other organisms attaining the ability to photosynthesize. These are called secondary events because the host cells consumed algae and incorporated the algal DNA into their own genomes, or genetic codes. Evidence suggests that red algae took part in at least four other engulfing events where their DNA was taken in by host cells. These resulted in modern-day diatoms and dinoflagellates.