In the endosymbiotic theory, what type of organism is thought to have evolved into chloroplasts?

In the endosymbiotic theory, chloroplasts are believed to have evolved from a type of bacteria, specifically a group known as cyanobacteria. This theory suggests that early eukaryotic cells engulfed these photosynthetic prokaryotes, which then formed a mutualistic relationship with their host cells. Over time, these engulfed cells became integral parts of the eukaryotic cell, evolving into chloroplasts.

Cyanobacteria are particularly significant because they are capable of photosynthesis, producing oxygen and organic compounds, which likely benefitted the host cell. The evidence supporting this theory includes similarities in the DNA of chloroplasts and cyanobacteria, double membranes surrounding chloroplasts, and the presence of ribosomes within chloroplasts that resemble those found in bacteria. This symbiotic relationship ultimately allowed for the diversification of plant life and contributed to the oxygenation of Earth's atmosphere.

Other options such as fungi, archaea, and viruses do not have the characteristics necessary to have evolved into chloroplasts. Fungi are eukaryotic organisms that do not perform photosynthesis, archaea are a separate domain of life with different biochemical pathways, and viruses do not have cellular structures or metabolic processes necessary to become organ