What is indicated by the fact that NOT all eukaryotic cells contain both mitochondria and chloroplasts?

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Endosymbiosis, the theory that explains the origin of organelles such as mitochondria and chloroplasts, suggests that these organelles originated from free-living prokaryotes that were engulfed by ancestral eukaryotic cells. The fact that not all eukaryotic cells contain both mitochondria and chloroplasts indicates that these endosymbiotic events occurred in stages, with mitochondria being acquired first in most eukaryotes and chloroplasts later in specific lineages that became photosynthetic.

What Is Indicated by the Fact That NOT All Eukaryotic