The origin of ethics
When animals want to organize themselves all they can think of is following a leader. Once a species has come into being that has developed a high enough level of communication so as to make it possible for individuals to make plans together, a far better organization model comes in sight. The group will have the best chances of survival if everybody's interest is taken equally into account. So a very strong instinct developed to see an equal in everyone you get in touch with; a sense of moral indignation when another fails to treat another as an equal; a sense of guilt when you realize you did that; and the thirst for revenge when a person fails to see it, the "an eye for a eye" principal forcing people in the position of the victim when they have failed to imagine themselves in that position.
It seems a watertight system but something must have gone wrong ... No theory explaining human properties can do without a variety of the story of the Fall.
We have to teach our children the knowledge of good and evil (or "values", as it's called nowadays), parents say. So they must have learned that from their parents, and they from theirs and so on ... So where did Adam and Eve get this knowledge? I don't know, it must have been growing on a tree or something ...
This must have been the starting point of the story of the Fall in the bible. There is only one way to teach your children the knowledge of good and evil: by making them feel guilty when they did something bad. They never do anything bad. Morality is part of human nature, but children have a development program, which includes the development of morality; you cannot expect the same social behavior from a child as from an adult. Interpreting children's behavior as bad is always misinterpretation. Original sin is making a child confess to a crime it hasn't committed.
The Fall is something that happens to practically everyone in this world, but not all people sell their souls to the devil by giving up on the principle of equality. In the bible the story is written with the pen of the devil, demonstrating the properties of the evil that is the result of that, sexism, inconsistency, absence of imagination ... incapacity to love.