Earth & Geography

How a single-celled organism almost wiped out life on Earth

There’s an organism that changed the world. It caused both the first mass extinction in Earth’s history and also paved the way for complex life. How? By sending the first free oxygen molecules into our atmosphere, and they did all this as single-celled life forms. They’re cyanobacteria, and the story of these simple organisms that don’t even have nuclei or any other organelles is a pivotal chapter in the story of life on Earth. Earth’s atmosphere wasn’t always the oxygen-rich mixture we breathe today. 3.5 billion years ago, the atmosphere was mostly nitrogen, carbon dioxide, and methane.

Almost all oxygen was locked up in molecules like water, not floating around in the air. The oceans were populated by anaerobic microbes. Those are simple, unicellular life forms that thrive without oxygen and get energy by scavenging what molecules they find. But somewhere between 2.5 and 3.5 billion years ago, one of these microbial species, probably floating on the surface of the ocean, evolved a new ability: photosynthesis. Structures in their cell membrane could harness the energy from sunlight to turn carbon dioxide and water into oxygen gas and sugars, which they could use for energy.

Those organisms were the ancestors of what we now call cyanobacteria. Their bluish color comes from the blue-green pigments that capture the sunlight they need. Photosynthesis gave those ancient bacteria a huge advantage over other species. They could now produce their own energy from an almost endless supply of raw ingredients, so their populations exploded and they started polluting the atmosphere with a new waste product: oxygen. At first, the trickle of extra oxygen was soaked up by chemical reactions with iron or decomposing cells, but after a few hundred million years, the cyanobacteria were producing oxygen faster than it could be absorbed, and the gas started building up in the atmosphere. That was a big problem for the rest of Earth’s inhabitants. Oxygen-rich air was actually toxic to them.

The result? About 2.5 billion years ago was a mass extinction of virtually all life on Earth, which barely spared the cyanobacteria. Geologists call this the Great Oxygenation Event, or even the Oxygen Catastrophe. That wasn’t the only problem. Methane had been acting as a potent greenhouse gas that kept the Earth warm, but now, the extra oxygen reacted with methane to form carbon dioxide and water, which don’t trap as much heat. The thinner atmospheric blanket caused Earth’s first, and possibly longest, ice age, the Huronian Glaciation. The planet was basically one giant snowball for several hundred million years. Eventually, life adjusted. Aerobic organisms, which can use oxygen for energy, started sopping up some of the excess gas in the atmosphere.

The oxygen concentration rose and fell until eventually it reached the approximate 21% we have today. And being able to use the chemical energy in oxygen gave organisms the boost they needed to diversify and evolve more complex forms. Cyanobacteria had a part to play in that story, too. Hundreds of millions of years ago, some other prehistoric microbe swallowed a cyanobacterium whole in a process called endosymbiosis. In doing so, that microbe acquired its own internal photosynthesis factory.

This was the ancestor of plant cells. And cyanobacteria became chloroplasts, the organelles that carry out photosynthesis today. Cyanobacteria are still around in almost every environment on Earth: oceans, fresh water, soil, antarctic rocks, sloth fur. They still pump oxygen into the atmosphere, and they also pull nitrogen out to fertilize the plants they helped create. We wouldn’t recognize life on Earth without them. But also thanks to them, we almost didn’t have life on Earth at all.

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