Final answer:
Endosymbiosis enabled the formation of early protists by incorporating photosynthetic prokaryotes into eukaryotic cells.
Explanation:
Endosymbiosis is the process that most likely enabled the formation of early protists. During primary endosymbiosis, a photosynthetic prokaryote was engulfed by a heterotrophic eukaryote, leading to the formation of organelles like chloroplasts. This process eventually allowed for the evolution of more complex eukaryotic cells.
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