@DennisVenema, in your defense, the question you laid out in Adam and the Genome was answered by your book. If “human” = Homo sapien, and we simplify the problem by ignoring the clear evidence of interbreeding, does that taxonomic category ever go to a single couple within the last 150,000 years? The answer, of course, is “no,” unless there was a lot of miracles or interbreeding. No one should be confused by that.
However, that is not the question being ask right now, and it appears it never was, at least not in this exchange.
Your are missing several critical things.
You have assumed without warrant that “humans” = Homo Sapiens. This is a minority position in the origins debate. Most YECs now day, for example, would say that Homo erectus is “human.” As we have seen recently, even OECs like @agauger are open to this position too.
Regarding Homo sapiens in particular, there is evidence of a large number of remains as anciently as 300 kya, not 200 kya. It is possible that Homo sapiens were around longer than to, say going back as far as 350 kya. So the short timeline of 150 kya is not really justified when considering Homo sapiens = “human” anyway. (This was discovered AFTER Adam and the Genome published, so @DennisVenema could not have included it in his book)
Also, there is very strong evidence that Neanderthals interbred with “humans,” as you define them. If that is the case, TMRCA estimates of Homo sapiens are going to be pushed back by that interbreeding event. So some sort of correction that excludes these parts of the genome is required. As I understand, no one has done studies like that in the literature.
Also, as I pointed out earlier, TMRCA tells us when things collapse to a single allele, not when they go down to 4 at the TMR4A. So using TMRCA’s on autosomal locations embeds the assumption that Adam and Eve were homozygotes and with identical genomes.
Neandertal’s Human or Not?
A fairly important question here to make sense of all this is if Neandertal are “human” too. If they are, there is a massive amount of additional variation that must be accounted for (pushing back estimates).
If they are not, some account has to be given for interbreeding. Do we accept it happened or not? How do we make theological sense of this in a way that avoids all the nastiness that so often arises in these conversations?
If we accept Neandertals interbred with “humans” (defined as Homo sapiens), then why would we think TMRCA would even give us a good estimate of “humans” in the past? It will always be giving us the sum total of Human plus Neandertal ancestors (and any other hominids Homo sapiens interbred with).
This is all to say, given the exceedingly strong evidence (including remains of a hybrid!) of Neanderthal + Homo sapien interbreeding, if Homo sapien = “human” the argument against a single couple origin of Homo sapien seems to weaken substantially. We would expect to see no single couple bottleneck, because population estimates are always adding in numbers from our Neanderthal ancestors.
The warrant of saying Homo sapien = “human” is very low any ways, so probably that is the part that should be dropped. But this is a big part of what is contributing to confusion about why @RichardBuggs and @agauger and many in the Church still have question after reading the book.
The case against a single couple origin (without interbreeding) has to be made in a way that does not assume Homo sapien = “human.” Honestly, the case against interbreeding with other lines is weak Scripturally, that very few people are going to insist on that any way.
Time to Most Recent 4 Alleles (TMR4A) and TMRCA
This last point about TMRCA and TMR4A is most interesting, and most likely to be abused. I’ve been thinking about this, but we should really see a distribution of TMRCAs of individual autosomal locations if there was a single couple origin.
A key overlooked parameters appears to be the NUMBER of OFFSPRING, that the first couple has. We will get very different distributions if they had 5, 10, 20 or 1000 kids (last on would clearly require miracles). However, in the very first few generations, some locations will very quickly drift to 3, 2, or 1 allele. The exact distribution will depend on precisely what happens in that moment, and even the degree of inbreeding, which will drive it lower. Someone needs to do the modeling here, but there may actually be a detectable variance increases in TMRCA measured in several locations.
The problem however, is that there is already high variance in TMRCA estimates. So if this shift in variance can be seen with any confidence, I am not sure. Regardless, this is a caution of applying my estimate that TMRCA / 4 → TMR4A when the whole genome is considered (as in the later studies). Those studies would not enable such a facile computation, or at least I do not think so.
However, the really interesting thing is the population structure in the first few generations of a single couple bottleneck. That appears to be an overlooked detail by @RichardBuggs and @agauger, and I am curious what they are thinking about that. Basically, we expect large linkage domains of the genome to have only one ancestral sequence, because of drift in the early population. That seems to undercut a major source of variation, and also suggest that this bottleneck should have been detected by now.
Our Common Ground
I do, however, want to emphasize that a very recent common couple ancestor (with no miraculous biology or interbreeding), say within 100,000 years ago, does appear to be ruled out. It is not as if @DennisVenema has missed something so large as to call that conclusion into question.
For those that think Adam and Eve are real, in our recent past, and our universal genealogical ancestors, ask if interbreeding with other lines is a problem in your reading of Genesis. If not, a genealogical Adam could have been recent (as recent as 10,000 years ago). To understand how that can be possible, you have to understand the difference between genetics and genealogy.