Who Keeps the Earth Running? — The Air You Are Breathing Was Made by Microbes

How to cook with sunlight

Long ago a bacterium lived in the sea: cyanobacteria. It worked out something new — catch sunlight, split water, and stick carbon dioxide from the air onto the pieces to make food.

Splitting water leaves something over: oxygen. To the cyanobacteria, oxygen was rubbish. They dumped it in the sea, and when the sea was full it spilled into the sky. For hundreds of millions of years.

As oxygen gathered above, something else formed. High up, sunlight turned some of it into the ozone layer — a roof against ultraviolet. Only once that roof existed could life leave the water and live on land. We stand on land thanks to that bacterium too.

A stranger living in my cells

Once oxygen was common, bacteria appeared that used it not as poison but as fuel, burning food with it for energy. Far more energy came out.

Then something very strange happened. A large cell swallowed one of those small bacteria and failed to digest it. The bacterium settled inside and took on the job of breathing; the large cell gave it room and food. Two became one.

Its descendants live in every cell of your body: mitochondria. They still keep their own genes and still multiply by splitting, like bacteria. Every breath you take, they are working.

Margulis, who first said this, had the paper rejected more than fifteen times; common sense held that a cell was one organism, not several joined. When genes could be read, the answer came: mitochondrial genes resembled a bacterium's more than their own cell's. So did the chloroplasts of plants — and chloroplasts are the descendants of those ancient cyanobacteria.

Moving air into the soil

There is more. Four fifths of the air is nitrogen, and life needs nitrogen to build bodies. But in air the two nitrogen atoms are bound so tightly that neither plants nor animals can use them. Thirsty in the middle of the ocean.

Only a few bacteria can prise them apart. Legume roots carry nodules, and inside the nodules live those bacteria. The plant feeds them; they pick nitrogen out of the air for the plant. That is why a bean field grows rich, and why farmers rotated beans long before anyone knew the reason.

People only imitated the trick with machines in the early twentieth century. That became fertiliser, which now feeds roughly half the world. A job bacteria have done for billions of years, copied a century ago.

Turning the dead into soil

Piece 2 said that without rot the world would stop. Now the full version.

Autumn leaves, fallen trees, dead animals: bacteria and fungi break them down and return them to soil and air. In that soil grass grows again, and animals eat the grass. If this stopped, forests would drown in leaves and the nutrients locked inside would never circulate to build new life.

A little further in

How briefly we have been here

Squeeze Earth's age into one day. Earth forms at midnight; cyanobacteria start making oxygen around six in the morning. Large creatures like us appear after nine at night. People arrive a few seconds before midnight.

Microbes lived almost the whole day, and through it they changed the air, made the soil and turned the water. We are the last tenants to move into a house they had already furnished.

Piece 1 said we only learned we had neighbours three hundred years ago. They were simply invisible; they were here from the start and they keep us alive now. That is what this series is called for. We have never lived alone.

Eight pieces, from a drop of water to the whole planet. And still there is what we have not seen: the crowds living without sunlight around hot vents on the sea floor, in salt lakes and Antarctic ice, and the countless others with no name yet. Most microbes cannot be grown in a laboratory, so we hardly know their faces.

The question that remainsIf they are the ones keeping this planet running — whose planet is it?