The endosymbiotic theory, proposed by Lynn Margulis in the 1960s, suggests that eukaryotic cells developed from the symbiotic relationship between different prokaryotic organisms. Synonyms for this theory include symbiogenesis, symbiotic theory, and symbiotic origin of eukaryotic cells. This theory has been widely accepted and is now considered a cornerstone of modern biology. It has helped us understand how complex cells evolved and how they formed the basis of all life on Earth. The endosymbiotic theory has also contributed to our understanding of the diversity of life and the interconnectedness of all living organisms. It remains a fascinating area of research and discovery for scientists today.