Its very hard to believe (or even prove) the environment they live in has been reletively constant all this time.
Their environment hasn’t remained constant. Rather, their biological forms are such that radical changes in the environment that might kill off other life forms completely, and/or push evolution in other life forms, don’t affect them as much. In other words they are like a solid boulder that remains on the landscape in a windstorm, while the pebbles and dust are blown around. Their forms are so well suited for survival that there’s nothing to “improve on” through most environmental changes.
Humans occupy a very unique place, evolutionarily speaking, because despite the relative “frailty” of our physical forms, we are able to do things that other animals can’t in order to survive environmental changes. We build different kinds of houses, wear different kinds of clothes, even adapt culturally to our surroundings. The adaptation that takes place physically in other animals takes place non-physically in us, with the notable yet biologically insignificant exception of racial modifications (which actually seem to cause very little alteration in the actual genetic code, as there are groups in Africa that live side by side and look quite similar, yet one of those groups might be much more close to Europeans, genetically, than to their neighbor). Perhaps as a result, humans have a very homogenous genetic code, meaning that the degree of genetic variation within our species is quite small compared to other individual species; the genes of a man from England and those of a Native Australian Aborigine are likely closer than two house-cats.
In short, both humans and crocodiles are able to survive relatively drastic changes. Humans by our mental adaptability, and sharks because of their relatively “rock-solid” forms.
But the fact remains if evolution is correct other Adams and Eves would have been evolving all around him, hence making Adam no longer unique and father of all.
This isn’t a necessary conclusion of evolution at all. The human soul can’t be “evolved”, it must be given by God directly, so there couldn’t be any other Adam and Eve’s “evolving”. From an evolutionary stand point, God could have simply allowed His world, working by His own design and transcendent causality (not requiring direct “efficient causality”, contrary to what Intelligent Design advocates insist) intervention to produce the precise physical form He wanted, and then directly and miraculously provided a new kind of soul to two members of this species. All true humans would have to be descended from these two, but there could be animals prior to them (and perhaps for a time afterwards) that were physically very similar, or even identical. No matter how physically similar these other animals might have been, they remained utterly distinct from true humans in a spiritual sense.
There is no need to appeal to “Intelligent Design”, nor to deny evolutionary theory outright, to come to this conclusion that keeps completely within Catholic orthodoxy and preserves everything required of Salvation theology.
As for a Thomistic treatment of the soul and body, I recommend questions
75,
76,
90, and
91 of the First Part of the Summa.
This article on “form” from the Catholic Encyclopedia might prove useful as well. Aquinas builds off of Aristotle’s “The Soul”, so if you can find a good summary of that it might help. Remember, though, that Aquinas’ views are generally dogmatic in themselves, although some of them have been dogmatized.
Peace and God bless!