Chaac was a human who either teamed up with the Sage of Stolen Flame to defeat the Volcano Lord, or was given the Sacred Flame to share with the people of Natlan after the fact, depending on which version of the story you read (Records of Hanan Pacha vs. Obsidian Codex).
He founded the first human tribe, and also made the first contact with the Wayob of the night realm.