This essay was Rousseaus's submission to the Academy of Dijon contest, entitled, "Has the progress of the arts and sciences contributed more to the corruption or purification of morals?".
This text is his story about Nature, and Society, and the scandal that happens when people come together, build, divide, dance, sing, and compare themselves with one another. In many ways, it is his answer to the problem of evil.
Natural man is, in many ways, good, because his needs are immediately felt and immediately fulfilled. Social man begins to compete, to hoard, and to use cunning to enslave his fellows, to gain their esteem, take their property, and sometimes their lives.
His picture of the natural man is half what we think of an "animal" and half the "human" that we recognize in ourselves. He shifts his description as the flow of arguement dictates. The habitual provocateur, Rousseau - watch him!
In a way, he is rewriting the Christian "Creation Myth". In his version, evil does not originate at that moment when man eats the fruit of the "Tree of Knowledge of Good and Evil" --to "be like God"; it happens when Adam wants a better apple than Eve's got for herself. Before society develops as we know it, Adam would have been fine with just a pear.