The Ecological Fallacy
Let’s start with this first claim by Dennis (rephrased by me and approved by Dennis).
On face value, we know that this cannot be known with certainty. Some scientists are not even sure if the remains earlier than 300 kya are fully Homo sapien. That means it is entirely reasonable to believe that Homo sapiens go to zero (less than a single couple) in this time frame.
Where is the error? The inference that (1) our ancestors never go below a few thousand, so therefore (2) Homo sapiens never go below a few thousand, is an example of the Ecological Fallacy. What applies to a group of things (the group here is “our ancestors”) does not necessarily apply to all the individuals in that group (the individual here is “Homo sapiens”).
To be 100% clear, this is not at all a challenge to mainstream population genetics, which makes claims about our ancestors as a whole. All the population size estimates (which are geometric averages over a time window) all include Homo sapiens + others, as far as I know. As far as I know, no one has found a way to figure out what the ratio is between the two population is; nor has anyone asked the question in a research study.
It is entirely possible that at sometime in the distant past, the total number of breeding Homo sapiens is precisely two, even though the total number of our ancestors at the time would be much more. Eventually, these early Homo sapiens would interbreed with others. This is an example of a hypothesis that (1) directly contradicts @DennisVenema’s claim, and (2) appears entirely consistent with the evidence, as I understand it (and I’m happy to be corrected). If any such hypothesis exists (and I think I’ve just demonstrated one), then Dennis’s claim is false.
The problem is not the findings of mainstream population genetics at all here. The problem is rather in the inference from this finding to the claim that this means Homo sapiens never dip below a few thousand. If we go back far enough their numbers go to zero. If they can go to zero without contradicting the evidence; so why can’t they go to 2? (of course, the would be interbreeding with surrounding non-sapiens).
Now, I stand to be corrected if there is a body of scientific work uncovered that cleverly demonstrates how Homo sapien DNA is separable from all other DNA, and effectively estimates population sizes over the last 350 kya years. It is entirely possible this large body of work exists, and I never encountered it.
Absent that body of work, I think Dennis just misunderstood (or misspoke) the relationship between the finding of population genetics and his novel claim, not even realizing his claim was novel (see Ecological Fallacy). If I had done the same, I would probably just retract the mistake. Perhaps replacing it with a more precise statement that actually does have a great deal of evidence. That’s just me though. I, personally, find no value in defending my own misstatements.
Everyone makes mistakes anyways. I find people trust me more when I am quick to retract mine.