re: Two lifeforms merge in once-in-a-billion-years evolutionary event

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that lets them do something that algae, and plants in general, can’t normally do – "fixing" nitrogen straight from the air, and combining it with other elements to create more useful compounds

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The first occurred about 2.2 billion years ago, when an archaea swallowed a bacterium that became the mitochondria.

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The phenomenon is called primary endosymbiosis, and it occurs when one microbial organism engulfs another, and starts using it like an internal organ. In exchange, the host cell provides nutrients, energy, protection and other benefits to the symbiote, until eventually it can no longer survive on its own and essentially ends up becoming an organ for the host – or what’s known as an organelle in microbial cells.

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The second time happened about 1.6 billion years ago, when some of these more advanced cells absorbed cyanobacteria that could harvest energy from sunlight. These became organelles called chloroplasts, which gave sunlight-harvesting abilities, as well as a fetching green color, to a group of lifeforms you might have heard of – plants. And now, scientists have discovered that it’s happening again. A species of algae called Braarudosphaera bigelowii was found to have engulfed a cyanobacterium that lets them do something that algae, and plants in general, can’t normally do – "fixing" nitrogen straight from the air, and combining it with other elements to create more useful compounds.

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In the 4-billion-odd-year history of life on Earth

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