If I am understanding Catholic teaching correctly – there is no “addition” of a spiritual soul. At conception, the [human] matter is animated by the God given spiritual soul. This precise moment is so profound “that one has to consider the soul to be the “form” of the body:” (CCC 365)
Then there are questions-- Is human DNA enough to produce true human nature? Is God still needed?
God is needed to provide the gift of a rational, immortal soul. Such a thing does not emerge naturally. Human DNA is not enough to produce a true human nature. That is, the rational powers of a human being are not just emergent qualities, but are a “leap” above any of the material components. [This is not to exclude the material brain from intelligence, computation, etc… but that there is something additional to a rational mind that even the best brain could not produce on its own.]
Animals are animated. In Saint Thomas’ thinking, they are a composition of matter and form, too. The form of all living things being called a soul, though lacking rational powers.
The form (as in hylemorphic dualism, referring to their essence, not shape or DNA) of these “proto-humans” and the form of true humans would be different. Proto-humans would be a type of animal only, whereas true humans are rational animals…
I would not imagine God changing the form of a proto-human mid-life (though with God, nothing is impossible). I’d imagine that Adam, if born from a proto-human, would have had from the moment of his conception the form of a “true human,” the form of a rational animal.
The proto-humans would have been still a very behaviorally complex and social animal, more than any other non-human animal today, but it would have lacked certain rational powers. In Saint Thomas Aquinas’ thinking, it would not have been able to grasp “universals” in its mind or engage in certain types of abstract thinking. They would have been reproductively compatible with the children of Adam and Eve, being similar in matter (physiology, DNA) even if different in form.