molecular evidence for endosymbiosis

Two of the most important structures inside your cells — the mitochondria that make your energy, and, in plants, the chloroplasts that capture sunlight — look oddly like little prisoners. They have their own DNA, divide on their own schedule, and are wrapped in a double membrane as if engulfed. The startling explanation, now strongly supported, is that they once were free-living bacteria, swallowed long ago by an ancestral cell and never digested — and molecular evidence is what turned this once-wild idea into mainstream fact.

The endosymbiotic theory says that more than a billion years ago, a host cell took in a free-living bacterium that, instead of being consumed, settled inside and became a permanent partner. The bacterium that became the mitochondrion was an aerobic bacterium good at using oxygen for energy; the one that became the chloroplast was a photosynthetic cyanobacterium. The strongest proof is molecular. Mitochondria and chloroplasts carry their own small circular genome, like a bacterium's, separate from the cell's nuclear DNA. Their ribosomes resemble bacterial ribosomes, not the host's, and are blocked by antibiotics that hit bacteria. And decisively, when you take the ribosomal RNA sequence of a mitochondrion and place it on the tree of life, it does not branch with its host cell — it branches deep among the bacteria, right next to its free-living relatives. The molecule remembers where it came from.

This is a showcase of molecular phylogenetics resolving a deep historical question that no fossil could settle, and it ranks among the most important events in the history of life — the merger that gave eukaryotic cells their powerhouses. A precise note: over time most of the original bacterial genes migrated to the host nucleus, so today these organelles keep only a tiny remnant genome and depend on the nucleus for most of their proteins. They are former bacteria, thoroughly integrated, no longer able to live on their own.