Final answer:
Lynn Margulis proposed the theory of Endosymbiosis in the 1960s, suggesting that organelles like mitochondria and chloroplasts were once free-living prokaryotic organisms.
Step-by-step explanation:
The theory of Endosymbiosis was proposed by Lynn Margulis in the 1960s. Endosymbiosis is the idea that certain organelles within cells, such as mitochondria and chloroplasts, were once free-living prokaryotic organisms that were engulfed by larger cells and developed a symbiotic relationship. Margulis provided evidence for this theory through her studies on the similarities between mitochondria and certain types of bacteria.
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