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Answer:
The Endosymbiotic Theory of eukaryote evolution is known as Symbiogenesis that states the evolution of eukaryotic cells from prokaryotic organisms.
The engulfing a functional cyanobacterium to produce photosynthetic eukaryotes ("plants") is considered as final step of the eukaryote evolution because a functional cyanobacterium closely resemble the chloroplast and related to mitochondria and it maintains a large genome that encodes thousands of proteins.
A photosynthetic eukaryotes ("plants") require cholorplast (green pigment) for photosynthesis and protein synthesis, which all are provided by functional cyanobacterium, after that a eukaryote can functions as an individual eukaryote and not as prokaryote, and hence engulfing functional cyanobacterium is considered as last step of eukaryote evolution.
