An old hunch about chloroplasts
The claim is a striking one about the origin of complex cells: that mitochondria, the structures that generate energy inside plant, animal and fungal cells, and chloroplasts, which carry out photosynthesis in plants, were not built up gradually from scratch but began as separate, free-living bacteria that were engulfed by another cell and never left. Botanists as early as Andreas Schimper in 1883 had noticed that chloroplasts divide in a manner resembling free-living cyanobacteria, and Konstantin Mereschkowski developed a fuller version of the idea in the early twentieth century, but it remained a marginal proposal with little supporting evidence for decades afterward.
Fifteen rejections and a 1967 paper
The modern case was made by Lynn Margulis, whose 1967 paper on the origin of complex cells assembled microbiological evidence for the theory in a way earlier proposals had not. The paper was turned down by around fifteen scientific journals before finally being accepted and published, and even after publication the theory remained a minority position within biology for roughly another decade, resisted partly because it implied a more radical picture of cell evolution, through merger and cooperation between organisms, than the gradual accumulation of small changes biologists were more accustomed to.
What the theory actually claims
The theory holds specifically that mitochondria descend from a group of bacteria called alphaproteobacteria and that chloroplasts descend from cyanobacteria, both originally independent, free-living organisms that became permanently incorporated into a host cell rather than being digested by it. Structural evidence supports this: both organelles are bounded by double membranes, consistent with having once been engulfed whole, both reproduce independently within the cell through bacterial-style division rather than being built fresh each generation, and both contain ribosomes that resemble bacterial ribosomes more closely than they resemble the ribosomes found elsewhere in the same eukaryotic cell.
The genetic evidence that settled it
The evidence that ultimately settled the debate came from genetics rather than structure. Beginning in the late 1970s, researchers including Robert Schwartz and Margaret Dayhoff showed that mitochondrial and chloroplast DNA is distinct from a cell’s own nuclear DNA and closely resembles bacterial genomes in its organisation, and further sequencing through the 1980s confirmed close evolutionary relationships between mitochondrial genomes and alphaproteobacteria, and between chloroplast genomes and cyanobacteria. This genetic confirmation is what moved the theory from a plausible but contested idea to the standard, widely accepted account of how eukaryotic cells acquired these organelles.
Why the organelles still carry some genes
What does not hold up as neatly is any complete account of why these organelles still carry their own, much-reduced genome rather than having transferred every gene to the cell’s nucleus over time, as most of their original genetic material clearly has been. Free-living cyanobacteria carry genomes with thousands of genes, while human mitochondria retain only a small fraction of that, a handful of dozens of genes, the rest having been lost or moved to the nucleus across evolutionary time. Precisely when the original merger occurred is also not settled: fossil evidence places it before roughly 1.6 billion years ago, and molecular estimates suggest somewhere around 1.8 to possibly over 2 billion years ago, but the exact timing and its relationship to rising atmospheric oxygen remain active research questions.
A vindicated theory, a more mixed later career
This case is worth attention for two reasons that pull in different directions. The scientific story is genuinely satisfying: a theory rejected repeatedly on first submission was later confirmed decisively by independent genetic evidence, and Margulis is rightly credited with pushing it through against sustained resistance. But her later career is a useful caution against treating a scientist’s authority as evidence in itself, since she went on to champion other, far more contested positions later in life that did not hold up to comparable scrutiny. The endosymbiotic theory earns its acceptance from the specific evidence behind it, not from who first argued for it, and that distinction is the real lesson here.