In 1966, a young biologist named Lynn Margulis submitted a paper to a scientific journal. It was rejected. She submitted it to another. Rejected again. She worked through a list of journals — the most prestigious, the most relevant, the most likely — and over the course of several years, she collected fifteen rejections. The reasons given varied. The idea was too speculative. The evidence was insufficient. The hypothesis was, according to some reviewers, simply too strange to be taken seriously.
The paper was eventually published in 1967 in the Journal of Theoretical Biology. Its title was “On the origin of mitosing cells.” It proposed, in carefully argued scientific language, one of the most radical ideas in the history of biology: that the cells of every complex organism on Earth — every animal, every plant, every fungus — are not singular entities, but ancient mergers. That the structures inside those cells, the ones that generate energy and capture sunlight, were once independent organisms that were absorbed, and never left.
The idea is now known as endosymbiotic theory. It is accepted as established fact. And it is stranger, when you really understand it, than almost anything in science fiction.
What lives inside you
To understand what Margulis was proposing, you need to start with mitochondria.
You have mitochondria in almost every cell in your body. They are the structures responsible for generating ATP — adenosine triphosphate — the molecule that your cells use as energy currency. Without mitochondria, your cells cannot power their own operations. Without that power, you cannot think, move, breathe, or pump blood. Mitochondria are, in the most literal sense, the engines of your existence.
They are also, Margulis argued, bacteria.
Not metaphorically. Not analogously. Descended from bacteria — ancient, free-living prokaryotes that were engulfed by a larger cell roughly two billion years ago, and that, instead of being digested, formed a cooperative relationship with their host. The host cell provided protection and nutrients. The engulfed bacterium provided efficient energy production. Both benefited. And over generations, the partnership became so entangled that neither could survive without the other.
The same story, Margulis argued, played out with chloroplasts — the green structures in plant cells that capture sunlight and convert it into chemical energy through photosynthesis. Chloroplasts, she said, are the descendants of cyanobacteria, a type of photosynthetic microbe that was similarly engulfed and co-opted, this time providing the ability to turn light into food.
Two ancient bacterial mergers. One for energy. One for photosynthesis. Together, they made complex life on Earth possible.
Bacteria existed before this merger, and they exist still. But every organism that followed — every animal, every plant, every fungus, every complex cell — traces its existence to this event. In that sense, endosymbiotic theory does not just explain a curiosity of cell biology. It explains the precondition for everything that makes the living world recognisable.
The evidence that convinced no one, then everyone
When Margulis first proposed this, the scientific establishment’s skepticism was not entirely unreasonable. The idea had actually been floated before, by botanists and biologists in the early twentieth century, and had largely been dismissed. The problem was not that it was unimaginable — it was that there was no compelling mechanism or evidence to support it.
Margulis changed that. She assembled a case built on multiple lines of converging evidence.
First, there was the question of mitochondrial DNA. Unlike almost every other component of a cell, mitochondria have their own DNA — a small, circular genome that is completely separate from the DNA in the cell’s nucleus. This circular shape is characteristic of bacteria. The DNA in the nucleus of your cells is linear. The DNA in your mitochondria is a ring, exactly as you’d find in a free-living prokaryote.
Second, mitochondria reproduce by binary fission — they split in two, the same way bacteria divide — not by the complex process of mitosis that eukaryotic cells use to reproduce. They divide on their own schedule, inside the cell, maintaining their own population.
Third, mitochondria are bounded by two membranes. The inner membrane has a composition and structure that closely resembles the membrane of a modern bacterium. The outer membrane looks more like the cell membrane of the host. This double boundary is exactly what you would expect if a bacterial cell had been engulfed whole.
Fourth — and perhaps most tellingly — there are antibiotics that specifically target bacterial ribosomes without affecting human cells. Those same antibiotics also affect mitochondria. Because mitochondrial ribosomes, unlike those in the rest of your cells, are bacterial.
Why it was rejected, and what that tells us
The fifteen rejections Margulis received reflect something deeper than institutional conservatism. At the time, the dominant view was that evolution worked through gradual, incremental change — through mutation and selection accumulating slowly over generations. The idea that a major evolutionary leap could happen through the wholesale absorption of one organism by another was not just unconventional. It proposed a different mechanism entirely.
The reviewers were not wrong to be skeptical. Science demands skepticism. The problem was that the skepticism was applied asymmetrically — the burden of proof placed on the radical new idea was considerably higher than it might have been for a more conventional hypothesis. Research has found that peer reviewers tend to apply stricter evidential standards to novel hypotheses than to incremental ones, a bias that has been documented in the scientific literature, and Margulis’s experience is one of its most famous examples.
It is also worth noting that Margulis became, in later life, something of a scientific contrarian — championing some ideas that did not hold up under scrutiny. The story of her great insight does not erase those complications. Science is done by complicated people.
The strangeness of what it means
Endosymbiotic theory, once you accept it, has implications that keep unfolding.
Consider: you are not, in a meaningful biological sense, a single organism. You are a community. Every one of your cells contains descendants of bacteria that made a deal with their host two billion years ago. Those bacteria still carry their own DNA. They still divide on their own schedule. They are, in a precise technical sense, a separate lineage — one that has become so intertwined with your existence that separation would mean death for both parties.
The mitochondria in your cells are not you. They are ancient guests who have been living in your lineage since before animals existed, before plants existed, before anything with more than one cell had ever appeared on Earth.
If the origin of complex life required this kind of symbiotic merger — if the eukaryotic cell is fundamentally a collaboration — then cooperation is not a secondary feature of evolution. It is written into the architecture of life at its most fundamental level. Every breath you take, every thought you have, runs on engines that your ancestors did not build, but absorbed.
The theory’s descendants
Since Margulis’s work was accepted, biologists have found additional examples of what are called secondary and tertiary endosymbiosis — cases where a cell that had already undergone one merger was itself absorbed by another. Some algae contain chloroplasts surrounded by three or four membranes, which is exactly what you would expect if a eukaryote containing a chloroplast was engulfed by another eukaryote.
The field of evolutionary biology has also come to appreciate that horizontal gene transfer — the movement of genes between organisms that are not in a parent-child relationship — is far more common than early evolutionists imagined. Bacteria swap genes routinely. Viruses insert their genetic material into host genomes. The tree of life is less a clean branching tree than a tangled web of connections and transfers.
Endosymbiotic theory was the first major crack in the strictly branching model. Margulis saw, before almost anyone else, that the history of life was not just a story of descent. It was a story of mergers, absorptions, and improbable partnerships that turned out to be the most consequential events in the history of biology.
Fifteen journals said no. The evidence said otherwise. And the Earth kept turning, billions of years of ancient collaboration quietly powering every living thing on its surface.
