Continuing the discussion from Heliocentric Certainty Against a Bottleneck of Two?:
I’ve been getting some questions on why I think a recent bottleneck (before 100 kya) is ruled out by the evidence. I noted several times in this series the reason why.
Mito- and Y-DNA
Let’s say (as most YECs do) that we rule out any scenarios that imagine Adam and Eve with very different biology than us (e.g. mosaics), or imagine God was constantly adding mutations to our genomes. A 50k bottleneck seems pretty well ruled out by the mitochondrial and Y-chromosomal data. Y- and mito- DNA has some very favorable properties in comparison to autosomal DNA for our purposes.
- There is no recombination, so these DNA stretches are much easier to analyze.
- Because of recombination, and perhaps by luck, neither of these are obviously contaminated by interbreeding.
- They are both are transmitted independently of one another.
- The mutation rates of both are well studied. They are directly measured in Y-Chromosome too, though this has not yet be done for Mitochondria (because it is really hard).
- One is transmitted exclusively by fathers, and the other exclusively by mothers, so they are independent of one another.
- Men/women have only one copy of y- and mito-genomes, so we do not need to worry about TMR4A vs TMRCA.
- We consistently get MRCA for y-chromosomal and mitochondrial genomes > 100 kya.
- Both YECs and others agree with each other on the source of variations in mitoDNA and yDNA (it is all mutations, and none of it is de novo created diversity).
So these are two essentially independent tests for a recent bottleneck. We also know many of the factors affecting increased mutation rates, and know they are not relevant on this timescale, and work differently for y and mito DNA. So this seems like a plausible bound. There will be debate on exactly where we drop down the line on what is ruled out by this evidence, but I do not think one can plausibly put a bottleneck at 50 kya. A very generous cut off seems to be 100 kya, though some will insist is must be 150 kya or even earlier.
For a 50 kya bottleneck (or a 6 kya bottleneck), there would have to be miracles and substantially different biology in Adam and Eve than we observe in humans today. One can believe that, but we would then be outside the realm of scientific inquiry.