There are really several interrelated questions here.
The first is whether it makes biological sense to speak of there having been “first members of the human race,” whom Scripture calls “Adam” and “Eve.” Generally speaking, there is no such thing as the “first member” or “first generation” of a biological species. For example, one cannot meaningfully speak of the “first horse” or the “first generation of horses.” Biological species typically arise through gradual changes over many generations, with no sharp boundaries between species along an evolutionary lineage.
The key to answering this question is to recognize that what defines a “human” being in the theological sense is not only a set of biological characteristics, but also the possession of an immortal “spiritual soul,” which is the basis of the human powers of reason and free will. Biological characteristics change gradually, but an immortal soul is something one either has or doesn’t have. And so, logically, there had to be a definite point where beings with immortal spiritual souls first made an appearance.
The Catholic view of human evolution, therefore, is that after a long and gradual process of biological evolution, which produced hominins who were highly advanced mentally, there was a sudden transition, in which God raised some of them to the “spiritual” level, i.e. to the level of rationality and freedom. Here is how the Vatican’s International Theological Commission described this in a 2004 document entitled Communion and Stewardship: Human Persons Created in the Image of God:1
“While the story of human origins is complex and subject to revision, physical anthropology and molecular biology combine to make a convincing case for the origin of the human species in Africa about 150,000 years ago in a humanoid population of common genetic lineage.” 2
“Acting indirectly through causal chains [i.e. of cosmic evolution and biological evolution] operating from the beginning of cosmic history, God prepared the way for what Pope John Paul II has called ‘an ontological leap ... the moment of transition to the spiritual … [i.e.] the special creation of the human soul … .’ ” 3
The second question is whether the first generation of beings who were “human” in the theological sense consisted of many individuals (an idea called “polygenism”) or just one couple (”monogenism”). The genetic evidence shows that humans emerged within an interbreeding population of at least a few thousand individuals (which is why the Vatican document quoted above refers to a “humanoid population”). The question, therefore, is whether in the “transition to the spiritual” God gave the gift of rationality and freedom to many and perhaps all of the thousands of Homo sapiens alive at that time --- making them all theologically human --- or did he do this initially for just one pair. In 1950, Pope Pius XII, in his encyclical letter Humani generis, warned Catholics not to “embrace” polygenism, because it was “in no way apparent” how polygenism is consistent with Catholic doctrines on “original sin.” 4 However, it is generally agreed that Pope Pius XII did not intend to definitively rule out the idea of polygenism, so it is an unresolved issue. It is worth noting that some well-known scientists have argued that the neurological basis for the brain’s processing of human language (a precondition for rational thought) must have originated with just one or a very few individuals. (See the book Why Only Us : Language and Evolution, by Berwick and Chomsky, in the “Resources for further study.”)
A third question concerns the sin committed by the first human beings (CCC 387-390), by which, according to Catholic doctrine, the human race became alienated from God and also subject to bodily death. How can human mortality be a consequence of that original sin, when we know that all animals are by nature mortal and that animals had been dying for hundreds of millions of years prior to the appearance of human beings? (As Ecclesiastes 3:19 says, “Surely the fate of human beings is like that of the animals; the same fate awaits them both: As one dies, so dies the other.”) There is no contradiction, however, for the traditional Catholic doctrine is that the first humans were offered bodily immortality for themselves and their descendants as a “preternatural gift” 5 (i.e. a gift that goes beyond what is natural) on the condition that they would not transgress the commandment God had given them. As that condition was not fulfilled, however, man became again subject to the bodily death that is the fate of all animals. In the view of St. Thomas Aquinas and other medieval Scholastic theologians, human nature in its present “fallen” state, is simply what human nature would have been if left to its own resources without the “preternatural gifts” and supernatural grace.
1.. Communion and Stewardship: Human Persons Created in the Image of God, International Theological Commission. http://www.vatican.va/roman_curia/congregations/cfaith/cti_documents/rc_con_cfaith_doc_20040723_communion-stewardship_en.html
2.. Communion and Stewardship: Human Persons Created in the Image of God, section 63.
3.. Communion and Stewardship: Human Persons Created in the Image of God, section 70.
4.. Humani generis (1950), Pope Pius XII, section 37. http://www.vatican.va/content/pius-xii/en/encyclicals/documents/hf_p-xii_enc_12081950_humani-generis.html
5.. St. Thomas Aquinas, Summa Theologiae, Part I, question 97. https://www3.nd.edu/~afreddos/summa-translation/Part%201/st1-ques97.pdf An accessible explanation of this can be found at http://readingthesumma.blogspot.com/2014/02/question-97-mans-physical-state-in.html
Resources for further study
St. John Paul II, Address to the Pontifical Academy of Sciences, October 22, 1996, “Magisterium is Concerned with the Question of Evolution for It Involves the Conception of Man." http://inters.org/John-Paul-II-Academy-Sciences-October-1996
Communion and Stewardship: Human Persons Created in the Image of God, International Theological Commission (chaired by Cardinal Ratzinger), July 23, 2004, sections. 62-69. http://www.vatican.va/roman_curia/congregations/cfaith/cti_documents/rc_con_cfaith_doc_20040723_communion-stewardship_en.html
Thomistic Evolution: A Catholic Approach to Understanding Evolution in the Light of Faith, Nicanor Austriaco, James Brent, Thomas Davenport and John Baptist Ku (Tacoma, WA: Cluny Media LLC, 2016), Chapters 25-28.
Faith, Science, and Reason: Theology on the Cutting Edge (2nd edition), Christopher T. Baglow (Midwest Theological Forum, 2019), Chapters 9-11.
Video: “Evolution and the Catholic Faith” (lecture by Stephen M. Barr, sponsored by the Lumen Christi Institute) https://www.youtube.com/watch?v=otSp_8otQ1s&t=2077s
Why Only Us: Language and Evolution, Robert C. Berwick and Noam Chomsky (MIT Press, 2015). (An accessible summary and review of this book is “First Words” by Stephen M. Barr, First Things (April 2017). https://www.firstthings.com/article/2017/04/first-words )