Serial Endosymbiotic Theory
Lynn Margulis · 1967
"The complex cells that make up plants, animals, and fungi arose when free-living bacteria were engulfed by other cells and, instead of being digested, survived as permanent internal partners — meaning every complex cell in your body is descended from an ancient merger between separate organisms."
Margulis's paper was rejected by roughly 15 journals before publication in 1967. Her claim sounded implausible: the power plants inside your cells used to be independent bacteria that got swallowed and never left.
Read more about the topic
The Selfish Gene
"Natural selection is best understood as operating at the level of the gene, not the individual or the species — organisms are best thought of as survival machines built by genes to make more copies of themselves."