Endosymbiotic theory explains how complex cells (eukaryotes) evolved when ancient, simple cells engulfed other simple cells, and instead of digesting them, the two organisms began living together permanently. Over vast stretches of time, the engulfed cells became the organelles we now call mitochondria and chloroplasts. This idea is one of the most important in all of biology, fundamentally reshaping how scientists understand the origin of all plants, animals, fungi, and other complex life on Earth.
How One Cell Became Part of Another
Roughly 2.7 billion years ago, after more than a billion years of prokaryotic (simple, single-celled) life on Earth, the first eukaryotic cells appeared. Endosymbiotic theory proposes that a large, host cell engulfed a smaller bacterium capable of using oxygen to produce energy. Rather than being broken down, the smaller cell survived inside the host. Both organisms benefited: the host gained a powerful internal energy source, and the engulfed bacterium gained protection and a steady supply of nutrients. Over millions of generations, the two became so dependent on each other that neither could survive alone. The engulfed bacterium eventually became what we know as the mitochondrion.
A second major event followed a similar pattern. A cell that already contained a mitochondrion engulfed a photosynthetic cyanobacterium, one capable of capturing sunlight and producing oxygen. That cyanobacterium became the chloroplast. This is why plants and algae can photosynthesize while animals cannot: only some lineages acquired that second symbiont.
Who Proposed the Theory
The idea has roots stretching back more than a century. In the early 1900s, a Russian botanist named Konstantin Mereschkowsky argued that plastids (the structures that include chloroplasts) were reduced cyanobacteria that had entered into a symbiosis with a host cell. He also proposed that the host itself was the product of an even earlier symbiosis between a large cell and a smaller one.
The theory languished for decades until 1967, when Lynn Margulis (then Lynn Sagan) published “On the Origin of Mitosing Cells” in the Journal of Theoretical Biology. Margulis didn’t just revive the idea. She expanded it, arguing that three fundamental organelles, mitochondria, photosynthetic plastids, and the structures underlying flagella (the whip-like tails some cells use to move), were all once free-living prokaryotic cells. The flagellum hypothesis never gained wide acceptance, but the case for mitochondria and chloroplasts proved overwhelming.
The Evidence: Why Scientists Accept It
Several independent lines of evidence converge to make endosymbiotic theory one of the best-supported ideas in evolutionary biology.
Double Membranes
Both mitochondria and chloroplasts are surrounded by two membranes. This is exactly what you’d expect if one cell engulfed another: the inner membrane would be the original membrane of the engulfed bacterium, and the outer membrane would come from the host cell’s engulfing process. Most other organelles in a cell have only a single membrane.
Their Own DNA
Mitochondria and chloroplasts carry their own small, circular genomes, separate from the DNA stored in the cell’s nucleus. This circular DNA structure is characteristic of bacteria, not eukaryotes. These organelles replicate their DNA and divide on their own schedule through a process resembling bacterial binary fission, where one organelle pinches in half to become two. While this division is coordinated with the rest of the cell, it mirrors how free-living bacteria reproduce.
Bacterial-Style Ribosomes
Ribosomes are the tiny molecular machines that build proteins. Eukaryotic cells have large 80S ribosomes in their cytoplasm, but the ribosomes inside mitochondria are much smaller, around 55S, closer in size and structure to bacterial ribosomes (70S) than to the cell’s own. This mismatch makes sense only if mitochondria descended from bacteria and retained their own protein-building machinery.
Genetic Evidence From Chloroplasts
Molecular studies confirm that the closest bacterial relatives of chloroplasts are cyanobacteria. Only cyanobacteria and chloroplasts share the ability to split water molecules using two photosystems to generate oxygen during photosynthesis. No other bacteria do this. Perhaps most strikingly, about 18% of the nuclear genes in the model plant Arabidopsis, roughly 4,500 genes, trace directly back to a cyanobacterial ancestor. Over evolutionary time, many genes from the engulfed organism migrated into the host cell’s nucleus, tethering the two together permanently.
Secondary Endosymbiosis: It Happened More Than Once
The story doesn’t stop with one round of engulfment. In secondary endosymbiosis, a eukaryote that already had chloroplasts was itself engulfed by another eukaryote. This produced organisms with chloroplasts wrapped in three or even four membranes instead of two, each extra membrane a trace of an additional engulfing event.
Several major groups of algae arose this way. Euglenophytes (the group that includes the familiar pond organism Euglena) have chloroplasts with three membranes, the result of a green alga being engulfed by a non-photosynthetic host. Chlorarachniophytes went through a similar event but retained even more of the engulfed alga’s cellular machinery, including a tiny remnant of its nucleus called a nucleomorph. Cryptophytes, another algal group, also contain a nucleomorph, this time from an engulfed red alga. These leftover nuclei are essentially fossils inside living cells, preserving a record of the merger.
Why It Matters Beyond Textbooks
The bacterial ancestry of mitochondria has surprisingly practical consequences, especially in medicine. Because mitochondria retained bacterial-type ribosomes, they are vulnerable to many of the same antibiotics designed to kill bacteria. Chloramphenicol, for instance, binds to the protein-building machinery in both bacteria and mitochondria, inhibiting protein synthesis in both. Tetracyclines, macrolides like azithromycin, and aminoglycosides all affect mitochondrial function to varying degrees.
This is why certain antibiotics carry side effects that seem unrelated to infection. Aminoglycosides can cause hearing loss, particularly in people with specific mutations in their mitochondrial DNA that make their mitochondrial ribosomes look even more like bacterial ones. The fatigue, muscle weakness, and other side effects sometimes associated with prolonged antibiotic use may partly reflect collateral damage to mitochondria. In a real sense, antibiotics can’t fully distinguish between the bacteria they’re meant to kill and the ancient bacteria living inside your own cells.
Endosymbiotic theory also reframes how we think about individuality in biology. Every cell in your body is a collaboration between at least two once-independent organisms. Every plant cell is a collaboration among three. Life at the cellular level is not a solo act but a partnership billions of years in the making.