2.2 billion years ago, a cell swallowed a bacterium that was very good at producing energy. This became the organelle we call mitochondria, the famed "powerhouse of the cell". The cell gave the mitochondria nutrients and protection, the mitochondria gave the cell energy.

1.6 billion years ago, a cell swallowed a bacterium that could turn sunlight into sugars. This became the organelle we call chloroplasts, and this is how we got plants. The cell gave the chloroplasts nutrients and protection, the chloroplasts gave the cells sugars to fuel growth and energy generation in mitochondria.

We call this process "primary endosymbiosis" and until recently, these were the only two such events we knew of. Now it seems like we've discovered a third.

Plants need nitrogen to grow. Nitrogen is abundant in the air, but very hard to extract. Some plants (most famously legumes) have developed symbiotic relationships with nitrogen-"fixing" bacteria to help get their bacteria.

100 million years ago, it seems like one plant, a type of algae called Braarudosphaera bigelowii (fun name) managed to pull off this swallow-but-not-digest trick with a nitrogen-fixing bacteria called UCYN-A, allowing the algae to fix its own nitrogen from the air. A once-in-a-billion-year evolutionary event!

Source https://newatlas.com/biology/life-merger-evolution-symbiosis-organelle/

In the picture, the chloroplasts are green, mitochondria purple and UCYN-A blue.

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