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'''Ab ovo''' (Latin: "from the egg") is a reference to one of the twin eggs of Leda and Zeus disguised as a swan from which Helen was born. Had Leda not laid the egg, Helen would not have been born, so Paris could not have eloped with her, so there would have been no Trojan War etc.
The English literary use of the phrase comes from Horace's ''Ars Poetica'', where he describes his ideal epic poet, who "does not begin the Trojan War from the double egg" (''nec gemino bellum Troianum orditur ab ouo''), the absolute beginning of events, the earliest possible chronological point, but snatches the listener into the middle of things (''in medias res'').