When I first reverted back to the faith and I wrestled with the generation and the procreation of man, I speculated that the children of Adam and Eve inter-married given that the entire human race comes from Adam and Eve (i.e., Adam and Eve probably had many daughters and sons beyond just the 3 that are named, i.e., Cain, Able, Seth, as Adam lived some 900 years).
Regarding souls, I think some distinctions should be noted. First, St. Thomas Aquinas taught that all life forms have a soul, i.e., an invisible “living principle”, that interacts with visible matter. Plants have a vegetative soul, capaciting plants to subsist, grow, and reproduce, i.e., “vegetative” faculties. Animals souls have this vegetative capacity, but their souls are also endowed with ability to move matter, (i.e. an “animative faculty”), and the ability to sense their surroundings, (i.e., “sensitive faculties or appetite”). I believe St. Thomas taught that upon death, the souls of plants and animals ceased to exist.
Adam and Eve were special-- not only did their souls have both vegetative and animative faculties, integrated with sensitive appetites, but their souls were also endowed with the crowning faculties of Intellect and Will-- which in the state of Original Justice, were perfectly integrated and goverened the lower faculties of the soul (vegetative, animative, sensitive). According to GOD’s good pleasure, the souls of Adam and Eve are IMMORTAL, just as ours are. However, after Original Sin, the higher faculties of Adam and Eve souls (i.e., intellect and will) no longer perfectly govern the lower faculites, particularly the vegetative (bodily death is now possible) and sensitive (concupiscence reigns in the flesh of man) faculties. After Original sin, the higher faculties also suffer loss, i.e., the INTELLECT is dimmed, and the WILL is weakened.
Only by GOD’s grace is our communion with GOD restored (sanctifying grace), and GOD capacitates us to reign in concupiscence, to know, love, and serve Him-- but with difficulty (a consequence of the fall). It will only be in heaven (and Christ’s 2nd coming) when our souls will be perfectly united with GOD, and thus capacitated to perfectly integrate all of our faculties, in union with a glorified body.
I wanted to interject some of St. Thomas’s ideas regarding what a soul or animating principle is. Thus, I did not address the question above. I believe some previous responses have addressed this question reasonably well (i.e., relatively early time in human civilization, compared to the number of people today that exist today, and needed development of culture/infrastructure to spread the good news to the ends of the earth). I am always in awe of GOD’s ways. HE very much likes to start out small. Then, with much time (with respect to our finite lives), HIS love, and willing helpers, HE cultivates that which HE has created. HIS creation grows immensely large in the space of time, until Christ’s 2nd coming.