Fall of man
In the beginning, God created elves corresponding to Adam and Eve of the Judeo-Christian creation myth, living in a perfect paradise. Through a snake in the grass, Satan tempted them to eat the fruit of the forbidden tree, whose poison caused a hallucination of mental clarity and inhibited genetic error correction mechanisms, leading to harmful mutations that formed the human race as we know it.
Fall in different worlds
In the Judeo-Christian account of real world events, Adam and Eve had no offspring before the fall, and all people carry the genetic traces of original sin. Jehovah's Witnesses often explain this with an analogy to a dent in a cake pan: every cake made with that pan will be dented.
Some faiths hold that sex itself was the original sin: the Unification Church believes that Adam and Eve sinned by having sex before reaching spiritual maturity, and Kundalini Yoga considers the "knowledge of good and evil" to refer to knowledge of what happens nine months after sex. However, Judaism and Christianity appear to contradict this, with the commandment to "be fruitful and multiply" preceding the commandment not to eat from that tree. (Genesis 1:28) In fact, God encourages married couples to find joy in sex.--Proverbs 5:15-19; Song of Solomon.
In the game world, they had sex and produced offspring. Only after having babies did they give in to the snake's temptation, eat the forbidden fruit, corrupt their genetic software, and set their de-evolution into motion. This forked the bloodline into two: those with the original sin, who became humans, and those without, who remained elves.