On the Origin of Mitosing Cells
Your cells' power plants were once free-living bacteria — swallowed, kept, and never let go.
The mitochondria powering your every heartbeat were, two billion years ago, free-living bacteria — and one day one of them moved in for good.
A cell inside a cell
Every complex living thing — you, an oak tree, a mushroom — is built from cells, and inside each of those cells live even smaller parts that were once independent bacteria. The mitochondrion, the tiny battery that turns your food and the air you breathe into usable energy, was one of them. So was the chloroplast, the green machine that lets a leaf eat sunlight.
The idea is that a big cell once swallowed a small bacterium and, instead of digesting it, kept it alive as a lodger. Over time the two became inseparable — one body, two genomes. Lynn Margulis argued that the most important cells on Earth are not single creatures at all, but old partnerships that never broke up.
The idea nobody wanted
When Lynn Sagan — she published under her married name before becoming known as Lynn Margulis — wrote this up in the mid-1960s, the notion sounded absurd to most biologists. An old, ridiculed idea, it was. She submitted the paper and, by her account, watched it bounce off about fifteen journals before the Journal of Theoretical Biology finally ran it in 1967.
Then it was mostly ignored — for nearly a decade. What rescued it was not rhetoric but evidence: once biologists could read DNA, they found that mitochondria and chloroplasts carry their own, and that it looks unmistakably bacterial. The heretic had been right.
Why it matters
Darwin had taught that new forms arise slowly, by small inherited changes sifted by survival. Margulis added a second, faster route to novelty: merger. Sometimes evolution does not tinker — it combines, fusing two whole organisms, and two whole genomes, into one. Without that single ancient merger there would be no animals, no plants, no fungi — no large life of any kind, because none of it could have harnessed oxygen the way mitochondria do.
The tenant who became family
Picture two companies that merge so completely they share one building, one payroll, one front door — yet each still keeps its own small filing cabinet of private records. The host cell is the building; the mitochondrion is the firm that moved in and stayed; and that little filing cabinet is the organelle's own DNA, the last proof that it once ran its own affairs. After a couple of billion years of living together, neither could survive being evicted.
Where it sits
Darwin (1859) gave life a single branching tree and a slow engine, natural selection; Avery and then Watson & Crick (1953) showed that heredity is written in DNA. Margulis added the twist that some of the branches on that tree did not just split — they grew back together and fused. Read alongside the Darwin and Watson–Crick entries in this Library, her paper completes a picture in which life advances both by dividing and, now and then, by joining.
A theory of the origin of eukaryotic cells (“higher” cells which divide by classical mitosis) is presented. By hypothesis, three fundamental organelles: the mitochondria, the photosynthetic plastids and the (9+2) basal bodies of flagella were themselves once free-living (prokaryotic) cells.
Classical mitosis evolved in protozoan-type cells millions of years after the evolution of photosynthesis.