One of the Tezcatlipocas (sons of the original dual god) transformed himself into the Sun and created the first humans to show up his brothers. The other gods, angry at his audacity, had Quetzalcoatl destroy the people, which he did with a flood. The people became fish. [Kramer]
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References: Kramer, Samuel Noah (ed.). Mythologies of the Ancient World, Anchor Books, Garden City, NY. 1961.