A number of distantly related groups of protists, as well as all plants, contain chloroplasts. The chloroplasts come in a number of different types; they may be surrounded by two, three, or four membranes, and they may contain different types of photosynthetic pigments.
Despite these differences, all chloroplasts trace back to the engulfment of an ancestral cyanobacterium by a larger eukaryotic cell—an event called primary endosymbiosis. In endosymbiosis, one organism lives inside another organism.
In the accompanying animation, we focus on this event of primary endosymbiosis and the subsequent evolution of chloroplasts in plants and several types of protists.
All chloroplasts trace their ancestry back to the engulfment of one cyanobacterium by a larger eukaryotic cell. This event—the step that gave rise to the photosynthetic eukaryotes—is known as primary endosymbiosis. The cyanobacterium, a Gram-negative bacterium, had both an inner and an outer membrane, in addition to a peptidoglycan layer in the cell wall.
The host cell membrane adds one more membrane during endocytosis. However, over evolutionary history, one of the three membranes was lost, leaving the first chloroplasts with two membranes.
The peptidoglycan layer has been subsequently lost from all but one group of microbial eukaryotes called glaucophytes.
It is almost certain that a single primary endosymbiotic event gave rise to the chloroplasts of the green and red algae. The photosynthetic land plants would then arise from a green algal ancestor. Almost all remaining photosynthetic microbial eukaryotes are the results of secondary or tertiary endosymbiosis.
In secondary endosymbiosis, a eukaryotic cell takes up a photosynthetic eukaryote, which subsequently evolves into a chloroplast. Over time, some groups have lost the engulfed cell's nucleus, while others have retained a trace of it. In some groups, four membranes surround this new chloroplast, while in others, such as euglenids, the cell membrane of the engulfed cell has been lost.
This evolutionary history explains why the euglenids have the same photosynthetic pigments as the land plants. It also accounts for the third membrane of the euglenid chloroplast.
In the accompanying animation, we examined the evolution of chloroplasts in a few groups of organisms.
Red algae, green algae, and land plants all have in common a similar type of chloroplast with two surrounding membranes. The two membranes derive from the inner and outer membranes of an ancestral cyanobacterium that was engulfed by an ancestral eukaryote. This primary endosymbiotic event—the formation of a photosynthetic eukaryote—is the origin of all chloroplasts.
The euglenids acquired their chloroplasts from a secondary endosymbiotic event. During euglenid evolution, an ancestral euglenid engulfed a photosynthetic eukaryote, kept the chloroplast, but lost the other elements of the ancient cell. From this history of events, the euglenid chloroplast has a third membrane.