Relatively large single-celled organisms such as amoeba often engulf and digest other, smaller micro-organisms. Sometimes they’re not digested and the smaller one just continues living inside the big one. This is a big deal because it’s how we got mitochondria, which are thought to be descended from free-living, parasitic, tiny bacteria. As we all know, mitochondria are the powerhouse of the cell, and indeed, without mitochondria, complex life as we know it could never have evolved. Chloroplasts also evolved this way.
There’s been a lot of debate about at what point the little bacterium on the inside stops being thought of as a bacterium and starts being more of an organelle, because it’s not just as simple as one organism inside another one, because symbiosis (a mutually beneficial relationship) isn’t a binary condition. One facet of symbiosis that I find cool is the exchange of genetic material — did you know that mitochondria have their own genome that’s much smaller than ours, and that mitochondrial genomes are inherited from your mum? Genes from the host cell can be transported to the little cell, and vice versa. They can become more optimised, just like how if you moved from a tiny, person flat into a fully equipped 5 bed family home, you might throw away your rusty tin opener.
Most of how this happened with mitochondria and chloroplasts is speculative because of how incredibly improbable this “endosymbiont event” would have been, but now we’re getting to see that gradient of endosymbiosis play out, this time with a nitrogen-fixing (captures nitrogen from the air) and an algae. It’s very cool, and I’m glad to learn of it.
If you find this stuff cool, one of my favourite pop-sci books I’ve ever read is Nick Lane’s “Power, Sex, Suicide: Mitochondria and the Meaning of Life” — I read it the year before going to uni to study biochemistry and it was accessible, but I learned a ton! Message me if you want a pdf link.