The endosymbiotic theory explains the origin of complex life by proposing that organelles within eukaryotic cells were once independent, free-living bacteria. This suggests an ancestral host cell engulfed these prokaryotic organisms, establishing a permanent, mutually beneficial relationship. This ancient merger gave rise to mitochondria, the cell’s powerhouses, and chloroplasts, the sites of photosynthesis. While the theory is a foundational principle of modern biology, its acceptance was marked by decades of dismissal involving multiple scientists.
Lynn Margulis and the Theory’s Modern Acceptance
The scientist most prominently associated with establishing the endosymbiotic theory in modern science is Lynn Margulis. In 1967, she published “On the Origin of Mitosing Cells,” synthesizing the long-dormant idea. Margulis, then signing her work as Lynn Sagan, proposed that the symbiotic association of once-free living prokaryotes drove the evolution of eukaryotic cells.
Her work was met with intense skepticism; the biology community largely favored the idea that organelles evolved in situ from the host cell’s internal membranes. The manuscript was reportedly rejected by approximately fifteen scientific journals before acceptance by the Journal of Theoretical Biology. This resistance stemmed partly from the prevailing focus on gradual, competitive evolution, which viewed large-scale mergers as improbable.
Margulis persevered, detailing how mitochondria originated from aerobic bacteria and chloroplasts from photosynthetic cyanobacteria. She provided a testable framework by meticulously compiling morphological, biochemical, and physiological evidence scattered throughout the literature. Her relentless advocacy and subsequent books, like Symbiosis in Cell Evolution, resurrected the hypothesis and presented it as a compelling scientific model.
The theory gained widespread acceptance in the late 1970s and early 1980s as new molecular data provided powerful substantiation for her claims. This evidence shifted the theory from a controversial hypothesis to a fundamental fact of evolutionary biology. Margulis’s contribution was the provision of the critical, modern scientific architecture that allowed the theory to finally become mainstream.
The Forgotten 19th Century Origins
The core concept of endosymbiosis significantly predates Lynn Margulis’s work, tracing back to naturalists in the 19th and early 20th centuries. The initial observation came from German botanist Andreas Schimper in 1883. Schimper noted that the division of chloroplasts appeared strikingly similar to the fission process observed in free-living bacteria. He proposed the idea that green plants originated from the union of a colorless organism and one tinged with chlorophyll.
Following this, Russian botanist Konstantin Mereschkowsky formally articulated the concept in 1905, naming it “symbiogenesis.” Mereschkowsky asserted that plastids, or chloroplasts, were descendants of cyanobacteria that had entered a symbiotic relationship with a larger host cell early in evolution. He was the first to argue that complex organisms arose through the permanent, intracellular union of two different kinds of cells.
The idea was extended to mitochondria in the 1920s by American biologist Ivan Wallin, who suggested that the organelle was derived from an aerobic bacterium. These early proposals failed to gain traction primarily due to the lack of molecular evidence. The technology to detect and analyze organelle DNA did not yet exist, and the prevailing scientific opinion assumed that the host cell’s nucleus controlled all cellular components. Consequently, without concrete genetic evidence, the ideas of Schimper, Mereschkowsky, and Wallin were largely dismissed and forgotten for decades.
The Overwhelming Biological Evidence
The endosymbiotic theory is now supported by evidence that links mitochondria and chloroplasts directly to their bacterial ancestors. One of the strongest pieces of proof lies in the genetic autonomy of the organelles. Both mitochondria and chloroplasts possess their own DNA, which is distinctly different from the linear DNA found in the host cell’s nucleus.
The DNA found within these organelles is circular, a structural characteristic shared with the genomes of bacteria and archaea. Furthermore, the protein-building machinery within these organelles strongly resembles that of prokaryotes. Specifically, mitochondria and chloroplasts contain 70S ribosomes, composed of 30S and 50S subunits, while the host eukaryotic cell uses larger 80S ribosomes.
Another compelling line of evidence is the unique double membrane structure surrounding the organelles, which is a direct relic of the original engulfment event. The inner membrane is thought to represent the plasma membrane of the ancestral bacterium, retaining bacterial features such as specific transport proteins and the presence of the lipid cardiolipin.
The outer membrane is believed to have been derived from the host cell’s engulfing vesicle, which initially wrapped around the ingested bacterium. The mechanisms of organelle proliferation also mirror those of their supposed ancestors. Mitochondria and chloroplasts reproduce through binary fission, a simple division method identical to how free-living bacteria multiply. This reproduction occurs independently of the host cell’s mitotic cycle, further cementing their evolutionary link to ancient prokaryotes.