You Are Descended From a Merger

In 1967 Lynn Margulis published a paper arguing that mitochondria, the structures inside your cells that release energy from food, were once free-living bacteria, engulfed by another cell and never digested. It was rejected by about fifteen journals before it appeared, and for years afterwards it was treated as an eccentricity.

The evidence is now overwhelming, and it is the kind that is hard to argue with. Mitochondria have their own DNA, circular, like a bacterium's rather than like the linear chromosomes in the nucleus. They have their own ribosomes, which resemble bacterial ribosomes closely enough that some antibiotics targeting bacteria also affect mitochondria. They are wrapped in a double membrane, which is what you would expect from something that was engulfed. And they divide on their own schedule, by splitting, rather than being manufactured by the cell.

The same story holds for chloroplasts in plants, which descend from photosynthetic cyanobacteria. Complex cells are not a gradual elaboration of simple ones. They are the result of a merger, and the mitochondria in the cells reading this sentence have their own ancestry stretching back to a separate organism.

What made the merger worth making is energy, and to see why, it helps to know what cells actually do with it.

The currency is ATP, and the crucial thing about it is that it is cash rather than savings. A human body contains only around fifty grams of ATP at any moment and turns over roughly its own body weight in it every day: it is made and spent continuously, never stockpiled. Its function is coupling: an energetically favourable reaction, the breaking of a bond in ATP, is harnessed to an unfavourable one so that the pair proceeds. Fat and glycogen are the savings account, and ATP is what the savings are converted into at the moment of spending.

How ATP gets made was one of the great puzzles of mid-century biochemistry, and everybody was looking for the wrong thing. The assumption was that there must be a high-energy chemical intermediate, and laboratories spent two decades hunting for it. In 1961 Peter Mitchell proposed that there was no such molecule: instead, energy from food is used to pump protons across a membrane, creating a gradient, and the protons flowing back through an enzyme physically rotate a molecular turbine that assembles ATP.

It was received with widespread disbelief, partly because it was a physical mechanism proposed in a field expecting a chemical one, and partly because Mitchell was working in a private laboratory in Cornwall funded largely by himself. It is correct. He received the Nobel Prize in 1978, and chemiosmosis turns out to run in mitochondria, chloroplasts and free-living bacteria alike, which is itself evidence for the endosymbiotic story, since it is the same machinery in all three.

Now the atmosphere, because the oxygen that makes this system so productive has a history, and it is not a pleasant one.

For the first two billion years or so of life on Earth, there was essentially no free oxygen in the atmosphere. Cyanobacteria evolved photosynthesis and began producing it as a waste product. For a very long time it was absorbed: by dissolved iron in the oceans, which precipitated out as the banded iron formations that are now mined, and by other mineral sinks. Around 2.4 billion years ago the sinks saturated and oxygen began to accumulate in the air.

To almost everything alive at the time, oxygen was a reactive poison. The Great Oxidation Event is probably the largest extinction in the planet's history, caused by pollution from an organism that had simply been doing what it did, on a timescale long enough that nothing could be said to have gone wrong. Anaerobic life was not eliminated but was driven into sediments, guts and deep rock, where an enormous amount of it still lives.

It also made everything afterwards possible, because aerobic respiration extracts far more energy from the same food than the anaerobic alternatives. Large, complex, energy-hungry organisms are only affordable in an oxygen atmosphere, and the cell that acquired a mitochondrion acquired the ability to use it.

One last detail about mitochondria, because it is widely quoted and just as widely over-read. Mitochondrial DNA is inherited only from the mother: the sperm's mitochondria are destroyed or excluded at fertilisation. Since it does not recombine, it preserves a clean unbroken maternal lineage, and mutations accumulate at a roughly steady rate, which makes it a good clock.

This is where "mitochondrial Eve" comes from: the most recent woman from whom every living person descends along an unbroken mother-to-daughter chain, who lived somewhere in the region of a hundred and fifty thousand years ago in Africa. She was not the only woman alive, she was not the first, and she is not the ancestor of anything else in your genome. Ten generations back you have more than a thousand ancestors, and mitochondrial DNA traces exactly one path through them. The popular reading of such results is almost always stronger than the result itself, which is a decent summary of this entire subject.