History of Symbiogenesis

Symbiogenesis denotes host and/or symbiont evolution through long-term symbiosis. Studied and defined multiple times over in the early 20th century by independently working scholars, including Famintzin, Mereschkowski, Kozo-Polyansky, Wallin, and Lederberg, symbiogenesis was brought to the attention of contemporary evolutionary science from the 1960s onward through the comprehensive works of Lynn Margulis who defined symbiogenesis as “the origin of a new organ, metabolic pathway, behavior, tissue, or other feature as a result of long-term hereditary symbiosis, [. which] includes the process by which organelles of the eukaryotic cell evolved from ancient bacterial symbionts.” Symbiogenesis is today wellrecognized for underlying the evolution of mitochondria and chloroplasts, which are eukaryotic cell organelles that originated from bacteria that engaged in endosymbiotic relationships with some of the earliest nucleated cells, some 2 billion years ago. Not confined to that period in time, endosymbiosis, or the phenomenon where organisms live inside other organisms (endosymbionts live inside a host), is quite common amongst eukaryotic organisms that often carry varied microbial communities in their body cavities and, albeit to a lesser extent, inside specific cells (intracellular endosymbiosis). This entry details the events that led to the formulation and wider academic recognition of symbiogenesis theory, its opposition to certain tenets of the Neodarwinian synthesis, and its integration into the emerging Reticulate Evolution school. As such, this entry presents a historical account of what was once a marginal theory that has now become essential for understanding the evolution of life.