Objection 1. It would seem that the human body was not produced by God immediately. For Augustine says (De Trin. iii, 4), that "corporeal things are disposed by God through the angels." But the human body was made of corporeal matter, as stated above (Article 1). Therefore it was produced by the instrumentality of the angels, and not immediately by God.
Objection 2. Further, whatever can be made by a created power, is not necessarily produced immediately by God. But the human body can be produced by the created power of a heavenly body; for even certain animals are produced from putrefaction by the active power of a heavenly body; and Albumazar says that man is not generated where heat and cold are extreme, but only in temperate regions. Therefore the human body was not necessarily produced immediately by God.
Objection 3. Further, nothing is made of corporeal matter except by some material change. But all corporeal change is caused by a movement of a heavenly body, which is the first movement. Therefore, since the human body was produced from corporeal matter, it seems that a heavenly body had part in its production.
Objection 4. Further, Augustine says (Gen. ad lit. vii, 24) that man's body was made during the work of the six days, according to the causal virtues which God inserted in corporeal creatures; and that afterwards it was actually produced. But what pre-exists in the corporeal creature by reason of causal virtues can be produced by some corporeal body. Therefore the human body was produced by some created power, and not immediately by God.
On the contrary, It is written (Sirach 17:1): "God created man out of the earth."
I answer that, The first formation of the human body could not be by the instrumentality of any created power, but was immediately from God. Some, indeed, supposed that the forms which are in corporeal matter are derived from some immaterial forms; but the Philosopher refutes this opinion (Metaph. vii), for the reason that forms cannot be made in themselves, but only in the composite, as we have explained (65, 4); and because the agent must be like its effect, it is not fitting that a pure form, not existing in matter, should produce a form which is in matter, and which form is only made by the fact that the composite is made. So a form which is in matter can only be the cause of another form that is in matter, according as composite is made by composite. Now God, though He is absolutely immaterial, can alone by His own power produce matter by creation: wherefore He alone can produce a form in matter, without the aid of any preceding material form. For this reason the angels cannot transform a body except by making use of something in the nature of a seed, as Augustine says (De Trin. iii, 19). Therefore as no pre-existing body has been formed whereby another body of the same species could be generated, the first human body was of necessity made immediately by God.
Reply to Objection 1. Although the angels are the ministers of God, as regards what He does in bodies, yet God does something in bodies beyond the angels' power, as, for instance, raising the dead, or giving sight to the blind: and by this power He formed the body of the first man from the slime of the earth. Nevertheless the angels could act as ministers in the formation of the body of the first man, in the same way as they will do at the last resurrection by collecting the dust.
Reply to Objection 2. Perfect animals, produced from seed, cannot be made by the sole power of a heavenly body, as Avicenna imagined; although the power of a heavenly body may assist by co-operation in the work of natural generation, as the Philosopher says (Phys. ii, 26), "man and the sun beget man from matter." For this reason, a place of moderate temperature is required for the production of man and other animals. But the power of heavenly bodies suffices for the production of some imperfect animals from properly disposed matter: for it is clear that more conditions are required to produce a perfect than an imperfect thing.
Reply to Objection 3. The movement of the heavens causes natural changes; but not changes that surpass the order of nature, and are caused by the Divine Power alone, as for the dead to be raised to life, or the blind to see: like to which also is the making of man from the slime of the earth.
Reply to Objection 4. An effect may be said to pre-exist in the causal virtues of creatures, in two ways.
First, both in active and in passive potentiality, so that not only can it be produced out of pre-existing matter, but also that some pre-existing creature can produce it.
Secondly, in passive potentiality only; that is, that out of pre-existing matter it can be produced by God. In this sense, according to Augustine, the human body pre-existed in the previous work in their causal virtues.
Source: Summa Theologica (New Advent)