Skip to content
NatureHQ

Claim check

Did mitochondria used to be bacteria?

Mostly supportedThe core is right; the usual phrasing overstates part of it.

The ancestry is right and among the best-evidenced historical claims in biology. The present tense is not: most of the ancestral genes now sit in your nucleus, and a mitochondrion cannot live independently of the cell.

The claim as it circulates

“The mitochondria in your cells were once free-living bacteria that got swallowed and never left — so you are carrying billions of captive bacteria around.”

Where you may have met it: Popular science writing and biology teaching; Social media facts about the human body; Documentaries about the origin of complex life

Taking it apart

What was claimed
That mitochondria are former free-living bacteria, taken up by an ancestral cell and retained — and, in the strong version, that they remain bacteria living inside our cells.
What was actually observed
Mitochondria carry their own small circular genome, divide by fission independently of nuclear division, are bounded by two membranes, use bacterial-type protein-making machinery and are correspondingly sensitive to certain antibiotics, and their genes group phylogenetically among bacteria rather than among their host’s. Later phylogenomic work placed the ancestor among the alphaproteobacteria and established that the great majority of ancestral genes have transferred to the host nucleus — a human mitochondrion retains thirty-seven, against a few thousand in a comparable free-living bacterium.
What the evidence supports
Bacterial ancestry, firmly. Five independent lines of evidence, resting on different kinds of data, agree — and each has a possible innocent explanation on its own that becomes implausible when all five hold of the same structure.
What it does not support
That a mitochondrion is a bacterium now. It cannot make most of its own proteins, cannot replicate outside a cell, and has been reduced and rebuilt across roughly two billion years. Some eukaryotes have taken the reduction further and retain organelles that no longer respire at all. The identity of the host cell at the time of the event also remains much less settled than the bacterial side.

This one is worth reading for how a historical claim gets settled at all. Nobody watched the event, and no single observation demonstrates it. What makes the case strong is that five different kinds of evidence — genomic, structural, pharmacological, cell-biological and phylogenetic — each point the same way for reasons that have nothing to do with each other.

The rest of the answer

The claims underneath

Each one carries its own evidence, scope and caveats. Expand any of them to reach the studies.

Mitochondria descend from free-living bacteria taken up by an ancestral cell. They are not bacteria now: most of their genes moved to the host nucleus, and none can live independently.

Established

Specialists would state this without hedging. Multiple independent lines of evidence agree.

Mitochondria are of bacterial ancestry, phylogenetically placed among the alphaproteobacteria. They retain a small remnant genome, with the large majority of ancestral genes transferred to the host nucleus, and are incapable of independent replication outside the cell.

Who this applies to
All eukaryotes, including lineages whose mitochondria are highly reduced.
Studied in
Eukaryota, Alphaproteobacteria
Why we rate it this way, and what the caveats are
EstablishedHigh confidence

Several independent lines converge: remnant genome, division, membrane structure, antibiotic sensitivity, and phylogenetic placement.

How far it can be extended

Established across eukaryotic diversity by phylogenomic evidence.

Caveats

  • The precise bacterial sister lineage is actively debated, and the nature of the host cell is much less constrained.
  • "Used to be bacteria" is right about ancestry and misleading about the present, which is the whole content of this claim.

Still unanswered

  • What the host cell was at the time of the event, which the bacterial evidence does not constrain.

Last reviewed 2026-09-04

The evidence (2 studies)

No single observation established endosymbiosis. Own DNA, own division, double membranes, bacterial-type ribosomes and phylogenetic placement each point the same way for separate reasons.

Established

Specialists would state this without hedging. Multiple independent lines of evidence agree.

The endosymbiotic origin of mitochondria and plastids is supported by independent lines: retention of an organellar genome, division by fission independent of nuclear division, envelope membrane structure, prokaryote-type translation machinery and antibiotic sensitivity, and phylogenetic placement of organellar genes among bacteria.

Who this applies to
Mitochondria and plastids across eukaryotes.
Studied in
Eukaryota
Why we rate it this way, and what the caveats are
EstablishedHigh confidence

Convergence of independent evidence types is what makes this among the better-supported historical claims in biology.

How far it can be extended

Each line of evidence has been examined across eukaryotic diversity.

Caveats

  • Independent lines can share a hidden assumption; here they rest on different kinds of data, which is what makes the convergence informative.
  • Some of the 1967 synthesis’s other proposals were not supported, and the case for organelles does not carry them.

Still unanswered

  • How many times plastid endosymbiosis occurred, and the details of secondary and tertiary events.

Last reviewed 2026-09-04

The evidence (2 studies)

Go deeper

Endosymbiosis

Your mitochondria have their own genome, divide on their own schedule, and are sensitive to antibiotics. Five separate clues, one answer.

Related things people ask

Last reviewed 2026-09-04