Life on Earth exhibits great variety, from the simplest single-celled organisms to complex multicellular beings. Unraveling how this diversity arose is a fundamental pursuit in biology. The endosymbiotic theory proposes a transformative event explaining the emergence of complex cellular life, particularly eukaryotic cells in plants, animals, and fungi. It suggests these intricate cells, with specialized internal compartments, resulted from ancient partnerships between simpler life forms, reshaping evolution.
What Endosymbiosis Means
Endosymbiosis describes a close and long-term biological interaction where one organism lives inside the body or cells of another. This relationship is often mutually beneficial. The term itself combines “endo,” meaning inside, and “symbiosis,” referring to organisms living together. This phenomenon is widespread in nature, ranging from nitrogen-fixing bacteria residing in plant roots to single-celled organisms hosting internal algae.
This partnership involves a larger host cell and a smaller cell, an endosymbiont, living within it. Rather than the host digesting the smaller cell, they form an enduring association. Over time, this cohabitation can lead to the endosymbiont becoming an integral part of the host cell, losing its independence. This process highlights how two distinct life forms can come together to create a new, more complex biological entity.
How Organelles Joined Forces
The endosymbiotic theory states that complex eukaryotic cells developed when an ancestral host cell engulfed prokaryotic cells, leading to the formation of mitochondria and chloroplasts. This process began when a larger host cell ingested an aerobic bacterium, a prokaryote that could use oxygen to generate energy. Instead of being digested, this bacterium survived within the host cell, establishing a mutually beneficial relationship.
The host gained an efficient energy-producing partner, as the bacterium became the precursor to modern mitochondria, providing energy through cellular respiration. In return, the bacterium received protection and a stable environment. This initial engulfment event is believed to have occurred early in the history of eukaryotes, as nearly all eukaryotic cells possess mitochondria.
Later, some of these early eukaryotic cells, already containing mitochondria, engulfed photosynthetic bacteria like cyanobacteria. These cyanobacteria, converting sunlight into food, evolved into chloroplasts. This second event provided the host cells with photosynthesis, leading to plants and algae. Both the host cell and the engulfed bacteria benefited, with the host gaining new metabolic capabilities and the endosymbionts receiving shelter and resources.
Supporting Evidence
Several lines of evidence support the endosymbiotic theory from striking similarities between mitochondria, chloroplasts, and free-living bacteria. Both organelles possess their own circular DNA, much like bacterial DNA and separate from the host cell’s nuclear DNA. This independent genetic material is passed down from parent organelles to offspring.
These organelles reproduce independently within the host cell through binary fission, identical to bacterial division. If removed, the cell cannot generate new ones; they must be inherited from pre-existing organelles. Mitochondria and chloroplasts also contain their own ribosomes, similar in size and structure to bacterial ribosomes, differing from the ribosomes found in the host cell’s cytoplasm.
Their membrane structure also provides evidence. Both mitochondria and chloroplasts are enclosed by two membranes. The inner membrane resembles bacterial cell membranes, while the outer membrane likely originated from the host cell’s engulfing membrane. These characteristics suggest that mitochondria and chloroplasts were once independent, free-living bacteria that formed a permanent association with ancestral host cells.