Look again at the opening of Genesis. "Zachar u-nekevah bara otam" — "male and female created He them" (Genesis 1:27). Why does the verse call the single creature otam, "them," if only one being had been made?
The rabbis noticed. And from this small grammatical oddity they grew a startling tradition: Adam was created with two natures fused in one body. Some sages said the male half stood on the right, the female on the left. Others said the two halves stood back to back, a single soul looking in two directions at once. Still others, with more poetic boldness, said Adam had a tail, and from that tail Eve was later fashioned.
The Targum Yonatan adds that Chavah was drawn from the thirteenth rib on Adam's right side. And the Midrash asks why a rib at all? Why not the head, or the eye, or the mouth, or the ear? Because, the sages answered, woman was not meant to be vain (she was not drawn from the head), nor wanton (not from the eye), nor a gossip (not from the mouth), nor an eavesdropper (not from the ear). She was drawn from a hidden, modest place — the rib — which lies close to the heart.
The humans we know came from a splitting. The lesson the rabbis drew is that male and female are halves of one original whole, and marriage is the long work of finding the other side of oneself again.