What would have happened had Eve sinned but Adam hadn’t? Would the whole human race still have perished? According to Anselm, the answer is “No.”
For God could have made another woman from Adam, in whom he had created the seed of all human beings; and by that other woman made from Adam, God’s purpose would have been fulfilled. (On the Virginal Conception, and On Original Sin ch. 9)
While Anselm does not go so far as to say that Adam’s second wife would have been taken from one of Adam’s other ribs (though this is surely more likely than not), he does assume that she would have been taken from Adam just as Eve had been. This is because, as I argued Anselm implies in Cur Deus Homo, to be human one must be taken from Adam. As the first human being, Adam was not merely an instance of a type in the divine mind, but was the very type itself. This means that to be human, one must be descended from Adam. To have the essence of humanity is not merely formal, but is genealogical.