How could Homer's epics survive without being written down?

Homer's epics survived because they were built to be sung, not read. Bards composed the Iliad and the Odyssey from a store of ready-made phrases called formulas, each one shaped to fit the rhythm of the line. By linking these pieces in familiar ways, a poet could compose verse rapidly, live, in front of an audience, and pass the tradition on to the next singer. This idea is called oral-formulaic composition.

Think of the phrases you know from Homer: "rosy fingered dawn", "winedark sea", "swift footed Achilles". They are not just decoration. Each one fills a precise slot in the hexameter, the six-part line of Greek epic. "Swift footed Achilles" even has the same rhythm as "glancing-helmed Hector", so a singer could swap heroes without breaking the line.

The man who proved this was Milman Parry, an American classicist later called the "Darwin of Homeric studies". He noticed that Homer says "divine Odysseus" or "much-enduring divine Odysseus" depending on how much room is left in the line, not on what is happening in the story. Then, between 1933 and 1935, he went to Yugoslavia to watch a living tradition. With his assistant Albert Lord, he recorded illiterate farmers singing enormous epics from memory. One singer, Avdo Međedović, performed a song of some 13,000 lines over five days. Parry said that listening to him, one felt one was "hearing Homer".

The poems were eventually written down, perhaps dictated by a singer to a scribe in the 6th century BC or earlier. But for generations before that, the formulas themselves were the memory.