We do not know how old some parts of the Sumerian poem of Gilgamesh, an ancient king of Uruk, are although it may belong to the 3rd millennium BCE. It was certainly rewrote or added to until about 700 BCE. Within it is contained Homer's Iliad, the Bible, and so much of subsequent literature.
The story concerns how to be the ideal king, the nature of life and dealing with death, gods and how to achieve autonomy from the gods and thereby true humanity, maturation, with the first clear instance of existentialism.
David Damrosch:
Gilgamesh is now very commonly assigned as a very first work in literature courses, and it shows a kind of early globalisation. It's the first work of world literature that circulates widely around the ancient world. The great thing about looking at Gilgamesh today is that we see that, if we go back far enough,there's no clash of civilisations between the Middle-East and the West. We find in Gilgamesh the origins of a common culture-its offshoots go off into Homer, 1001 nights, and the Bible-so it is really a sort of common thread in our global culture.