Whether or not primary chloroplasts came from a single endosymbiotic event, or many independent engulfments across various eukaryotic lineages, has long been debated. It is now generally held that organisms with primary chloroplasts share a single ancestor that took in a cyanobacterium 600–2000 million years ago. It has been proposed this the closest living relative of this bacterium is "Gloeomargarita lithophora." The exception is the amoeboid "Paulinella chromatophora", which descends from an ancestor that took in a "Prochlorococcus" cyanobacterium 90–500 million years ago.