Final answer:
Chloroplasts originate from ancestral photosynthetic bacteria in endosymbiotic theory. They entered the plant cell lineage through primary endosymbiosis, contributing to the evolution of eukaryotes.
Step-by-step explanation:
Chloroplasts are thought to have originated from ancestral photosynthetic bacteria that lived symbiotically inside a eukaryotic cell. This is known as endosymbiotic theory, proposed by Konstantin Mereschkowski. The first articulation of this theory suggested that chloroplasts and the nucleus of plant cells both originated from bacteria. Over time, primary endosymbiosis led to the establishment of chloroplasts within the plant cell lineage, contributing to the evolution of eukaryotes.