Nicene and Post-Nicene Fathers, Vol. II:
City of God: Chapter 21

Chapter 21.—That There Was Created at First But One Individual, and that the Human Race Was Created in Him.

Now that we have solved, as well as we could, this very difficult question about the eternal God creating new things, without any novelty of will, it is easy to see how much better it is that God was pleased to produce the human race from the one individual whom He created, than if He had originated it in several men.  For as to the other animals, He created some solitary, and naturally seeking lonely places,—as the eagles, kites, lions, wolves, and such like; others gregarious, which herd together, and prefer to live in company,—as pigeons, starlings, stags, and little fallow deer, and the like:  but neither class did He cause to be propagated from individuals, but called into being several at once.  Man, on the other hand, whose nature was to be a mean between the angelic and bestial, He created in such sort, that if he remained in subjection to His Creator as his rightful Lord, and piously kept His commandments, he should pass into the company of the angels, and obtain, without the intervention of death, 565 a blessed and endless immortality; but if he offended the Lord his God by a proud and disobedient use of his free will, he should become subject to death, and live as the beasts do,—the slave of appetite, and doomed to eternal punishment after death.  And therefore God created only one single man, not, certainly, that he might be a solitary, bereft of all society, but that by this means the unity of society and the bond of concord might be more effectually commended to him, men being bound together not only by similarity of nature, but by family affection.  And indeed He did not even create the woman that was to be given him as his wife, as he created the man, but created her out of the man, that the whole human race might derive from one man.

Footnotes

Coquaeus remarks that this is levelled against the Pelagians.