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This particular instance started 100 million years ago, so it's not a right-place-right-moment situation.

There are only two known instances of symbiogenesis occurring, mitochondria and plastids (which includes chloroplasts), which justifies the once in a billion years description. Other instances are suspected. https://en.wikipedia.org/wiki/Symbiogenesis

There are lots of known endosymbionts, separate organisms living inside the body or cells of another, a prerequisite for symbiogenesis. https://en.wikipedia.org/wiki/Endosymbiont

> Altogether, the team says this indicates UCYN-A is a full organelle, which is given the name of nitroplast. It appears that this began to evolve around 100 million years ago, which sounds like an incredibly long time but is a blink of an eye compared to mitochondria and chloroplasts.

It happened 100 million year ago, not in the petri dish at the lab.

It really has only happened a few times in the history of life, at least only a few times that survived. The title could use a little work though.