'In the 1920s, an American professor named Ivan Wallin (1883–1969) published a string of articles and a book about mitochondria. Wallin’s day job was teaching anatomy to medical students at the University of Colorado, but he was also fascinated by cell biology. Over the course of a decade, Wallin forged the concept of “symbionticism,” whereby speciation in animals and plants is triggered by the acquisition of bacterial symbionts. Wallin believed that mitochondria were such symbionts and had evidence to prove it—his publications included camera lucida drawings of “mitochondria” growing happily on agar plates!'

https://journals.plos.org/plosbiology/article?id=10.1371/journal.pbio.3002581