If you took a man, and magically created a woman out of this man’s rib, then got the two to mate and produce children, then took these children and made them have sex with their own siblings through incest, and the children’s children were made to do the same, quickly, rapidly, enough genetic abnormalities will accumulate to not allow this arrangement to last more than a few generations. Definitely not enough to create 7 billion humans alive today, or the billions who died before them. If the starting pair had been somehow genetically “pure” enough to allow this, then why aren’t we seeing a generational decay in humans today? People are, in fact more or less more healthy today than a thousand years ago.
Pardon me here, but just for the sake of devil’s advocacy, there is divine intervention to account for. Don’t get me wrong, I agree that human genetics does not point to a common ancestry within the last ten thousand years; that’s not in question. I’m just pointing out, if someone is going to use divine intervention as an explanation for the origin of man, then nothing is really stopping them from using divine intervention to explain man’s genetic diversity.
By way of example, I’ve heard the following explanations from creationists at one time or another:
* God created other men and women directly for the descendents of Adam and Eve to marry, and that’s where the genetic diversity comes from - yes, this doesn’t solve all the population issues, and I don’t honestly know if there’s biblical support for it; it’s a less common argument.
* Humans were genetically “perfect” in Eden, and with the fall came genetic diversity - now, the genetic record certainly doesn’t support this one, as there does not appear to be such a thing as “perfect genetics” now or in our history, but the fall makes for a fairly typical supernatural event people can cite to say “this is where it all changed”. It’s not a convincing argument on a scientific level, but sufficient appologetics if the base theology is accepted.
* God specifically tweaked or edited the first X generations of humans after they left the garden (in some versions, “cursed” man with these tweaks), or after certain biblical events, and that amounts for the differences between then and now - this sort of claim will sometimes claim that that’s the reason that biblical figures were so long lived; that humans still were in a different or “higher” state for a time after Eden, and it’s only later generations that got the tweak. This sort of extended timeline with the “tweaking” is typically also used to get past the genetic bottleneck that Noah’s story implies. Again, not terribly convincing from a scientific standpoint, but apologetically reasonable.
And your point is???