Stasinos, in the epic poem called the Little Iliad, does not rush in medias res, but begins with the eggs of Leda, from one of which Helen was born. If Leda had not laid this egg, Helen would never have been born. If Helen had not been born, Paris could not have eloped with her. If Paris had not eloped with Helen, there would have been no Trojan War, etc.
Ab ovo usque ad mala.
A Roman coena (dinner) consisted of three parts. The first course was the appetiser, and consisted chiefly of eggs, with stimulants; the second was the “dinner proper;” and the third the dessert, at which mala (i.e., all sorts of apples, pears, quinces, pomegranates, and so on) formed the most conspicuous part. —Hor. Sat. I. iii. 5.
Source: Dictionary of Phrase and Fable, E. Cobham Brewer, 1894
More on Ab from Fact Monster: