Endosymbiosis is a biological relationship in which one organism lives within the body or cells of another organism, forming a mutually beneficial partnership. This concept was first proposed by scientist Lynn Margulis in the 1960s to explain the origins of eukaryotic cells, which are believed to have evolved from prokaryotic organisms through a process of symbiosis. One of the most well-known examples of endosymbiosis is the relationship between mitochondria and their host cells. Mitochondria are thought to have originated as free-living bacteria that were engulfed by an ancestral eukaryotic cell and eventually evolved into an organelle responsible for generating energy through aerobic respiration. Endosymbiosis is a fundamental concept in evolutionary biology and has been studied extensively to understand the coevolution of different species and the mechanisms underlying the establishment and maintenance of symbiotic relationships. Researchers continue to investigate the molecular and genetic aspects of endosymbiosis in various organisms to gain insights into the evolution of complex life forms and the diversity of life on Earth.