The Endosymbiotic Theory (EST) describes one of the most significant evolutionary events in the history of life: the origin of complex cells. This theory proposes that the internal energy-producing structures within today’s complex cells, known as mitochondria and chloroplasts, were once independent, free-living bacteria. A larger primitive cell engulfed these smaller prokaryotic cells, forming a permanent partnership. Instead of being digested, the smaller cells survived inside the host, providing specialized functions that offered a massive survival advantage to the combined organism. This cellular merger explains how simple cells transitioned into the much larger and more complex eukaryotic cells that make up all animals, plants, fungi, and protists.
The Early Conceptual Foundations
In the early 1900s, Russian botanist Konstantin Mereschkowsky was the first to formally outline the concept he called “symbiogenesis.” Mereschkowsky proposed that plastids, the structures that hold chlorophyll in plant cells, were derived from ancient, free-living cyanobacteria. The American biologist Ivan Wallin further developed this hypothesis in the 1920s, specifically arguing for the bacterial origin of mitochondria. Wallin believed that mitochondria were descendants of endosymbiotic bacteria and even attempted to culture them outside of their host cells. However, his work and Mereschkowsky’s ideas were widely dismissed by the scientific community of the time. Critics often viewed these proposals as speculative because they lacked concrete, testable evidence. The technology of the era did not allow scientists to closely examine the internal genetic material of organelles.
Lynn Margulis and the Theory’s Modern Acceptance
American biologist Lynn Margulis was the pivotal figure responsible for reviving and popularizing the Endosymbiotic Theory, first publishing her ideas in 1967. Margulis’s contribution synthesized the old ideas of Mereschkowsky and Wallin with new data from the emerging field of molecular biology. By the 1960s, scientists had discovered that mitochondria and chloroplasts possessed their own genetic material, separate from the cell’s main nucleus. This discovery provided the molecular evidence needed to support the century-old morphological observations. She argued that the similarities between these organelles and bacteria were too numerous and specific to be coincidental. Margulis proposed that the acquisition of an aerobic bacterium—the ancestor of the mitochondrion—was the defining step in the evolution of all non-plant eukaryotes. Following this initial merger, she posited that the ancestors of plant cells later acquired a photosynthetic cyanobacterium, which became the chloroplast. Despite initial skepticism, Margulis persistently championed the theory, integrating evidence from electron microscopy, biochemistry, and genetics. Her comprehensive synthesis forced the scientific community to reconsider the old hypothesis.
Biological Evidence Supporting the Theory
The acceptance of the Endosymbiotic Theory rests on multiple lines of biological evidence showing that mitochondria and chloroplasts share distinct traits with free-living bacteria. One compelling piece of evidence is the presence of their own genetic material. Both organelles contain a single, circular DNA molecule that is structurally similar to the chromosome found in prokaryotic cells, rather than the linear DNA strands found in the host cell’s nucleus. These organelles also reproduce independently of the host cell through a process called binary fission, which is the exact method used by bacteria to divide. Furthermore, if a cell loses its mitochondria or chloroplasts, it cannot regenerate them from scratch; it must inherit them from a parent cell. The structure of their membranes provides further support for the engulfment hypothesis. Both mitochondria and chloroplasts are enclosed by two distinct layers of membrane. The inner membrane contains specialized transport proteins and molecules characteristic of a bacterial cell membrane. In contrast, the outer membrane is thought to have been derived from the host cell’s membrane that enveloped the bacterium during the initial engulfment event. Finally, the protein-building structures within the organelles, known as ribosomes, are also characteristic of bacteria. Mitochondria and chloroplasts contain 70S ribosomes, which are smaller than the 80S ribosomes found in the cytoplasm of the host eukaryotic cell.