The paper that explained why every living thing on Earth exists was rejected by 15 journals before anyone took it seriously — and the idea it contained is stranger than most science fiction

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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...