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 o

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 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 https://en.wikipedia.org/wiki/Endosymbiont