We should remember that, in classical Catholic theology, Adam and Eve need not have been naturally incapable of dying. The Catechism explicitly says that “even though man’s nature is mortal, God had destined him not to die” (CCC 1008). In other words, immortality was a divine gift, not necessarily an intrinsic biological property.
So without the Fall, Adam and Eve would not have undergone physical death, even though human nature considered in itself was mortal: through sin, the original gift preserving humanity from death was lost, and natural mortality became the actual condition of fallen humanity.
That is also quite compatible with Genesis 3:22–24, where continued access to the tree of life is what would allow man to “live forever.”
What the Genealogical Adam model does not by itself answer is the theological status of human contemporaries who were not yet descended from Adam, that requires an additional theological account, as universal genealogical ancestry alone does not solve it.
And this is where I think Kenneth Kemp’s model helps us. His mofrl begins with a population of hominins that are biologically capable of supporting rational life but are not yet fully human in the philosophical/theological sense because they do not possess rational souls. God then creates and infuses rational souls into an original pair (Adam and Eve ) and gives them the preternatural gifts associated with original justice. They are therefore the first genuinely rational human persons.
Their descendants are also given rational souls by God, some of those descendants can interbreed with the surrounding biologically similar population, and God can create rational souls for the resulting offspring. In that way the biological population remains large ( exactly as population genetics requires ) while every theological human person ultimately descends from the original rational pair.
Kemp explicitly summarizes his position as:
- Adam and Eve were the first rational beings
- and all other rational beings were descendants of Adam and Eve.
This also gives a straightforward answer to the mortality problem: biological death can exist for millions of years among animals and pre-human hominins without being the “human death” involved in Romans 5. Adam and Eve possess naturally mortal bodies, but God grants the first human persons a supernatural or preternatural preservation from death. When they fall, that gift is lost, and their genuinely human descendants inherit human nature without that original gift.
There is also a more phenomenological argument, so to speak, from human nature: we tend to experience death not simply as a biological fact, but as something that should not be there, almost as though the soul recognizes “the last enemy” before the mind is even capable of articulating why.
I have mentioned this episode on the forum before. My only clear memory from the first five or six years of my life is from when I was about two and first discovered that people could die. I had not yet received any religious education, and I had not even grasped the universal character of death (I certainly had not yet understood that I myself and my loved ones would necessarily die one day, my parents hadn’t told me that yet, at the time). Yet I remember vividly sitting in my high chair and crying inconsolably. My reaction was simply one of rejection: this is wrong; this should not happen.
My mother later (many years later) told me that, to calm me down, they took me to a pharmacy, had the doctor show me an antibiotic, and told me that thanks to that medicine people did not die anymore. Apparently, that was what finally reassured me. 
I don’t remember that part (although I obviously have no reason to think my mother lied), I just remember me discovering death and sitting in my high chair crying.
Obviously, this proves nothing by itself, but I do find it philosophically suggestive. At two years old I was hardly capable of constructing any kind of rational argument, I had not even understood death as a universal fate, and yet the rejection was already there.
That is why Paul’s description of death as “the last enemy” has always struck me as so right. It is almost as though something in us recognizes the enemy before our brains are developed enough to explain what, exactly, we are objecting to.
I wouldn’t present that as a proof of the Fall, but I do think it fits remarkably well with the Christian claim that, although human beings are naturally mortal creatures, death is not the final end for which the human person was made and we instinctively know that, we know it shouldn’t happen, we know it’s not how things were supposed to be.