Sometimes the Tree of Life is represented (in a combination with similar concepts such as the World Tree and Axis mundi or "World Axis") by a staff such as those used by shamans. Examples of such staffs featuring coiled snakes in mythology are the caduceus of Hermes, the Rod of Asclepius, the Staff of Moses, and the papyrus reeds and deity poles entwined by a single serpent Wadjet, dating to earlier than 3000 BCE. The oldest known representation of "two" snakes entwined around a rod is that of the Sumerian fertility god Ningizzida, who was sometimes depicted as a serpent with a human head, eventually becoming a god of healing and magic. It is the companion of Dumuzi (Tammuz), with whom it stood at the gate of heaven. In the Louvre, there is a famous green steatite vase carved for King Gudea of Lagash (dated variously 2200–2025 BCE) with an inscription dedicated to Ningizzida. Ningizzida was the ancestor of Gilgamesh, who, according to the epic, dived to the bottom of the waters to retrieve the plant of life. But while he rested from his labor, a serpent came and ate the plant. The snake became immortal, and Gilgamesh was destined to die.