EVOLUTION × GENETICS × LIFE

Endosymbiosis and the Mitochondrion

~1.9 billion years ago (order of events debated)

Proterozoic · Paleoproterozoic · Orosirian

The mitochondrion, a compartment of complex cells, began as a bacterium that came to live inside another cell about 1.9 billion years ago and transferred most of its genes to that cell's nucleus.

Endosymbiosis is the state in which one organism lives durably inside another. A bacterium came to live inside another cell and remained there. Over generations, the bacterium transferred most of its genome, the full set of its genes, to the nucleus of the host cell. It retained a small chromosome of its own and became the mitochondrion, the compartment in which complex cells burn oxygen. That chromosome is still present in every one of our cells, and researchers use it to identify the origin of the mitochondrion. The host was an archaeon, a single-celled organism of a group distinct from bacteria. The bacterium was an alphaproteobacterium, a member of one large group of bacteria, or it belonged to a lineage that diverged just before that group.

Why it matters

The entry of a bacterium into another cell, from which the mitochondrion came, occurred only once. Organisms of visible size could not exist without it.

Dating & uncertaintyDisputed

The endosymbiosis left no fossil. Researchers study it by phylogenomics, which builds family trees from genomes. They read it in the branch lengths of the proteins the bacterium brought: a branch is longer when a protein has changed more. The analyses differ on the order of events. Either the mitochondrion entered a simple host and drove its later changes, or it joined an already complex cell.

Earth history as one day

13:57:2110 h 3 min before midnight

if Earth’s 4.54 billion years were compressed into a single day