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.
So, here’s the thing. I get it that you’re attempting to use this to prove that evolution is false, but that’s not what the text is saying. A theory of evolution wasn’t in play in Aquinas’ day, so he can’t be arguing what you’d like him to be arguing.
Let’s look at his argument again: it doesn’t attempt to say that a material form cannot have made a human. Instead, it’s arguing against the proposition that
some other immaterial form created humans. That’s what Aquinas is arguing against. (In fact, he implicitly assents to the principle that, at a later date, scientists argued: “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”. That’s an argument
for evolution! (Although I wouldn’t claim that Aquinas isn’t arguing evolution, but is unwittingly providing the grounds from which one
can argue for evolution!))
So, Aquinas is simply pointing out that no other immaterial form created the first human being from which another human could be generated, and therefore, the creation of man belongs to God alone.