Naturaly, if you want to describe some abstract reality to somebody completely untouched by knowledge of it you use physical terms.
The snake represent spiral-like force which is in subtle physical, known in east as kundalini power and in the west it is quite often use as symbol for medicine. This pranic force(cosmic energy) is climbing in spiral along the spine and spinal chord (Tree of life) and upon reaching the human brain awakens the functions of intelect which happens in the case of human beings and it marks significant step in natural evolution. However with intelect comes possibility of living in the ego and possibility of sin which forces man out of living in the tune with Nature and Cosmic Laws (Garden of Eden).
So no fairy Tree of life, no real Snake and no actual Garden of Eden. Its all human aproximation.