How did Hesiod give the Greek gods a family tree?

Hesiod gave the Greek gods their family tree in a poem called the Theogony, composed around 730 to 700 BC. The name means "the birth of the gods", and the poem tells how the world began and how its gods came to be, generation by generation. It is the first known Greek account of how the cosmos began, and ancient authors credited Hesiod and Homer with establishing Greek religious customs.

The story starts with Chaos, a dark void, then Gaia (Earth), Tartarus and Eros (Desire). From there the gods multiply through a series of overthrows. Cronus overthrew his father Uranus. Because it had been prophesied that one of his own children would overthrow him, Cronus swallowed each child his wife Rhea bore, until she handed him a stone wrapped in baby's clothes instead of Zeus. Zeus then overthrew Cronus and the Titans and became the permanent ruler of the cosmos.

Zeus even shut the cycle down. He swallowed his first wife Metis while she was pregnant with Athena, because she was fated to bear a son who might overthrow him.

He tells us he was pasturing sheep on Mount Helicon when the Muses presented him with a laurel staff, a symbol of poetic authority, and his land at Ascra caused lawsuits with his brother Perses. He is regarded as the first written poet in the Western tradition to see himself as an individual with an active role in his subject.

The Theogony is not the one true version of the myths: it is a snapshot of a tradition that happened to crystallise when Hesiod set down the stories he knew.