A ancient metaphorical explanation for the phenomenon of consciousness in pre-human organisms, forever separating humans from their animal (non-sapient) ancestors.
Many mythologies in geographically separated cultures feature trees in a central role, such as Yggdrasil, the Norse World Tree, Ahuehuete, the Aztec Tree of Life, the Buddhist Bo Tree, and Disneyland's Tree of Life in the Animal Kingdom.
Carl Sagan wrote an interesting treatment of this metaphor in The Dragons of Eden, a book about the roots and evolution of consciousness.