It has been proposed that the serpent was a symbol of rebirth because of its ability to shed its old skin and appear as a reborn snake. It may have been a representation of the earth and underworld with each head representing one.
Historically, serpents and snakes represent fertility or a creative life force. As snakes shed their skin through sloughing, they are symbols of rebirth, transformation, immortality, and healing. The ouroboros is a symbol of eternity and continual renewal of life.