I think the story in Genesis is an allegory for when humankind became self-aware. That same self-awareness lead to knowledge of our own mortality beyond our immediate survival instinct. That is where the eating of the tree also made it so that we "would surely die". Before that point, while perhaps intelligent, we were true innocents, animals who simply acted on sophisticated instinct or to external stimuli. Eating of the tree was in effect the very first choice humankind ever made, their first action against instinct, which led to them becoming aware of themselves and their bodies, and feeling shame over them. Incidentally, my personal belief is that it is the fear of death that drives us to commit most evils. We fear death and will do whatever we can to put it off by making ourselves as secure as possible, which of course leads to greed, war, all the rest of it.
Incidentally, in many other cultures and traditions serpents are revered as bringers of wisdom and knowledge, the being who taught mankind of civilization, of mathematics and astronomy and technology and the like. The feathered serpent Quetzalcoatl in Mayan mythology, for example, or the Caduceus (intertwined snakes) the god Hermes carried, and which today is a symbol for medicine and healing. Even Christianity has such a tradition, albeit split across two different storylines. The one half is the serpent in the garden, the other half is Enoch, the father of Noah. The Books of Enoch were never included in the Bible, though they form part of the Ethiopian Canon, but Enoch is represented as the father of civilization, being taught the arts of smithing and mathematics and many other sciences by God. Interestingly, Enoch is usually equated with Hermes, who was the god of invention for the Greeks, and Thoth, the Egyptian god of wisdom. In alchemical and occult circles they are syncretised as Hermes Trismegistus, Hermes the Thrice Great, who brought Wisdom, in either of the three guises, to all three the original cultures of the ancient world.
Mina.