The Power Stations in Your Cells Used to Be Free-Living Bacteria

Inside many of your cells are tiny structures that carry their own DNA, separate from the DNA in the cell's nucleus. They are the descendants of bacteria that were swallowed more than a billion years ago, never got digested, and never left.

Somewhere in your body right now, a liver cell is going about its business. Inside it, there may be more than 2,000 tiny structures called mitochondria.

Their job is energy. They produce ATP, the molecule that cells use as their energy currency. That's why mitochondria have the nickname "the powerhouse of the cell".

But they have a peculiar feature. Each one carries its own DNA. Not part of the main set of instructions stored in the cell's nucleus, but a separate genome of its own, usually arranged as a single little circle.

Which raises an obvious question. Why would one part of a cell have its own private set of genes?

The answer is that mitochondria didn't start out as part of the cell at all.

The guest that stayed

Here's the idea. A very long time ago, one cell engulfed a bacterium. Normally that would be the end of the bacterium. It would be digested.

This time, it wasn't. The bacterium survived inside its host. And instead of being food, it became a partner. It stayed, it multiplied, and its descendants were passed down from generation to generation of host cells, until eventually they became the mitochondria in your cells today.

The theory is called symbiogenesis, and it's now the leading explanation for how complex cells, the kind with a nucleus that make up plants, animals and fungi, came about. It says that mitochondria, and some other structures inside cells, descend from free-living bacteria that were taken up by a host.

It's not a new idea. The biologist Konstantin Mereschkowski sketched out the theory in 1905. In the 1920s, Ivan Wallin argued for a bacterial origin of mitochondria. But it was the biologist Lynn Margulis who, in 1967, built the case and backed it up with microbiological evidence.

The tell-tale clues

So what's the evidence that your mitochondria were once bacteria? It's a bit like spotting someone who moved countries as a child. Years later, they've settled in completely, but there are still little giveaways in the accent.

First, that DNA. Mitochondrial DNA is circular, much like the chromosomes of bacteria.

Second, the way they reproduce. Mitochondria multiply by dividing in two, the way bacteria do.

Third, their protein-making machinery. Cells build proteins using tiny machines called ribosomes. The ribosomes inside mitochondria are more like bacterial ribosomes than the ones in the rest of your cell. And the proteins they make begin with the same chemical starting signal that bacterial proteins use.

Fourth, the membranes. Mitochondria have a double membrane, an inner layer and an outer layer, and those layers contain molecules, called porins and cardiolipin, that are also found in bacteria.

And finally, the family resemblance. When scientists compare mitochondrial genes with those of living bacteria, they point to one particular group, the alphaproteobacteria.

Any one of those clues might be a coincidence. All of them together are hard to explain any other way.

Moving in for good

Here's the counterintuitive part. If mitochondria are former bacteria, why can't they just leave and live on their own again?

Because over a vast stretch of time, most of the guest's genes moved house. Most of the original bacterium's genes were either lost or transferred into the host's nucleus. Today, a typical mitochondrion relies on up to a thousand or more proteins whose genes sit in the nucleus, not in the mitochondrion itself.

In humans, the mitochondrion's own genome is down to just 37 genes, only 13 of which code for proteins. And the transfer is still going on: copies of mitochondrial DNA keep turning up stitched into the DNA of the nucleus.

So neither partner can now manage without the other. The mitochondrion can't survive outside the cell, because most of its instructions are filed somewhere else. And the cell relies on the mitochondria to power it. What started as a guest has become, in effect, an organ.

It happened again

That isn't the only time this has happened.

Plants have mitochondria too, but they also have chloroplasts, the structures that capture sunlight and carry out photosynthesis. And chloroplasts appear to have exactly the same kind of backstory. They evolved from cyanobacteria, a group of bacteria that photosynthesise, through a single endosymbiotic event around two billion years ago.

Like mitochondria, chloroplasts have their own DNA, separate from the nucleus, and they multiply by dividing. And the pigments inside them, chlorophyll a and chlorophyll b, are what give them their green colour.

Which means that every green leaf you've ever seen is green because, a very long time ago, one cell swallowed a photosynthesising bacterium and didn't digest it.

What we still don't know

None of this means the story is complete. Some big questions are still argued over.

When did the first merger happen? It must have been more than about 1.45 billion years ago, because that's the age of the oldest undisputed fossils of complex cells. Beyond that, estimates vary widely.

What was the host? One view holds that it was already a fairly complex cell with a nucleus, which engulfed the bacterium. Another holds that the host was a much simpler cell, an archaeon, a member of a separate branch of single-celled life. Exactly which bacterium became the mitochondrion, and in what order the events unfolded, also remain unresolved.

A very old partnership

So the next time you take a breath, spare a thought for the tiny power stations at work inside you: descendants of bacteria that moved in more than a billion years ago and never left.

In a sense, you're a partnership. And it's a partnership that has been running for more than a billion years.

SOURCES

1. Wikipedia, "Symbiogenesis." https://en.wikipedia.org/wiki/Symbiogenesis (accessed 25 Oct 2026)

2. Nature Education (Scitable), "The Origin of Mitochondria." https://www.nature.com/scitable/topicpage/the-origin-of-mitochondria-14232356/ (accessed 25 Oct 2026)

3. Wikipedia, "Chloroplast." https://en.wikipedia.org/wiki/Chloroplast (accessed 25 Oct 2026)

4. Wikipedia, "Mitochondrion." https://en.wikipedia.org/wiki/Mitochondrion (accessed 25 Oct 2026)