First, there was never a bottleneck of two people that accounts for the diversity of humans living today.
Second, geneticists tell us that the diversity of contemporary humans derives from no less than 12,500 black African ancestors, 2,500 of whom left Africa to populate the rest of the globe between 50,000 and 100,000 years ago. That’s as narrow as the bottleneck ever gets.
Third, the probability that Y-chromosome Adam (120,000-350,000 years ago) and mitochondrial Eve (140,000-200,000 years ago) were of the same reproductive age at the exact same window in time, were located close to one another geographically, and actually had children together, is vanishingly small. (If you want to chuck a miracle into your first-couple thesis, I suppose that would be the place to do it.)
But, in any case, there was no first couple. There was not even a first human. There is only the continuum of an evolutionary lineage that goes all the way back to the first cell.
In other words, like thumbing slowly from one page of a cartoon flip book to another, a single offspring does not tend to dramatically vary from its parent.
Just as one cannot pinpoint the moment when a tadpole becomes a frog, or a toddler a child, so there was no moment that a Homo heidelbergensis couple gave birth to the first “true” Homo sapien. These classifications are for our convenience, but at the boundaries they’re not meaningful. If you’re going to refer to the “first” Homo sapien, he or she spent its first nine months in the womb of a Homo heidelbergensis mother, and could just as easily have remained designated as one of those as one of us.
Introducing the miraculous insertion of souls into a first couple, and positing that that first couple’s offspring slowly displaced the soulless, doesn’t help, for their genetic markers would still accompany them, and geneticists say that our contemporary genetic diversity is too large to be explained by a bottleneck of two people. There never was such a dramatic, two-person, genetic bottleneck.
So Occam’s razor here suggests something much simpler than the bestiality hypothesis: Genesis 2 and 3 should not be read literally.
Like the Iliad, the Genesis story has beauty, poetry, and psychological power, but strictly speaking, it’s not true. There was no first man formed from inorganic dust, no first woman drawn from his rib, no special garden between the Tigris and Euphrates rivers where they lived, no forbidden tree they ate from by which death and sin entered the world, and no children they produced that went on to colonize the planet.
It’s okay to treat an etiological narrative as an etiological narrative. It’s okay to correct a genre category mistake (mistaking a figurative narrative for a literal one).
Occam’s razor, baby, Occam’s razor.