NO SINGLE EVENT. “Margulis, a champion of endosymbiotic theory, argued for the community perspective on the eukaryotic condition. Symbiotic mergers similar to eukaryogenesis, she insisted, are ubiquitous in the history of life. As a result, our understanding of evolutionary processes requires the radical revision, perhaps abandonment, of many traditional neo-Darwinian ideas.”
http://www.pnas.org/content/112/33/10278.full