<https://en.wikipedia.org/wiki/Symbiogenesis>
Mitochondria were sufficiently established in general awareness to be a plot point of Madeleine L'Engle's A Wrinkle in Time, being a concept which fascinated the character of Charles Wallace Murray, a young prodigy, in the stories. Though I don't recall if symbiogenesis is specifically referenced. The first story in the series was published in 1962.