@glipsnort we are going far afield from the original goal into speculative territory.
The question, as I see it, is if a brief bottleneck is ruled out in the distant past by evidence.
As for the likelihood this, it will be strongly shaped by our prior beliefs, because there is not evidence to tell us one way or another.
Some people come to the table already believing that there was single couple origin. Some of us will dispute whether that is warranted (I certainly do), but that is their starting point based on their reading of Scripture. I do not think there is good reason to think there as a single couple origin from the scientific data alone. It seems like that is what you are asking for, but that data does not be possible to find.
“Explosive” growth is a highly biased term in this context. We are merely talking about the difference between 3 (observed in hunter gatherer societies) vs. 4 (what I initially said) children per couple, for just a couple centuries. This is hardly an absurd amount of growth, clearly in the realm of plausibility. We can look for the sensitivity of coalescence methods (PSMC and MSMC) to detecting tight bottlenecks with different growth rates afterwards. TMR4A is at about 500 kya, but I expect that number to move upward to about 600 to 700 kya with a growth rate you would find acceptable (e.g. 0.4% vs. 1%, or 3 children per couple instead of 4).
As for what could cause this?
That is irrelevant to the point. We are talking about what the data shows us. I can speculate totally untested natural mechanisms for a single couple origin, and have. However that is just speculative and not supported by data. I never claimed it was. There is not data here to discriminate.
People who think this is important for theological reasons, I am sure, are not making a scientific point. They would see that the data does not rule out a single couple origin during this time. Perhaps God made a couple and isolated them from their neighbors. We are not really talking science here. Once trust in that position will be entirely determined by theology (and not contradicted by evidence), but it is not my personal view, so do not ask me to justify it theologically.
I think large part of the confusion here is understanding how theological questions should negotiated the line between science and not-science.
From a purely scientific view, I think we can say:
There does not appear to be evidence against a brief single couple bottleneck before about 500 kya, but that might be pushed back to about 700 kya or so.
There does not appear to be evidence for such a bottleneck either.
We cannot find good reason to think from a purely scientific point of view why precisely a single couple bottleneck would be expected, but there may be some reasons a small bottleneck might have taken place (e.g. natural disaster, etc.)
However, now thinking from a theological point of view, engaged with this science, we can take this further, beyond the science.
- Perhaps God created the first humans as a single couple, sole-genetic progenitors of us all, perhaps 2 mya.
Is that ad-hoc? Well, I’m not convinced by it, but I also cannot argue from evidence that it is wrong. If other’s feel it is not ad hoc, they can take that position. It really is not for science to say one way or another where the evidence is silent.
That is all I’m saying.