In the very beginning of the world, Adam and Eve sinned and the evil that was merely potential in the Garden became a matter of conscious choice for all following generations. Cain was the first child of Adam and Eve. The Midrash tells us that he was born with a twin sister whom he married. We know about Cain - his name and something of the significance of his life. But about the first female born on earth we know nothing, not even her name. I have pictured Cain and his sister/bride entwined in utero. Midrash claims that Cain was born before Adam had repented, and therefore the impurity of the serpent clung to him, imbuing him with an impulse for evil even before he entered the world. Cain brought murder to the world when he killed his brother Abel, but the first female to be born on earth brought us the power of generation. She is shown casting a zygote, symbolizing her gift of fertility. (http://janetshafner.com/biblical-visions/Cain+and+His+Sister-Bride.jpg.php)