Geneticist Richard Buggs, Reader in Evolutionary Genomics at Queen Mary University of London, has just written an intriguing article in Nature: Ecology and Evolution (28 October 2017), titled, Adam and Eve: a tested hypothesis? Comments on a recent book chapter. It appears that Buggs is unpersuaded that science has ruled out Adam and Eve. He thinks it’s still theoretically possible that the human race once passed through a short, sharp population bottleneck of just two individuals, followed by exponential population growth. Buggs disagrees with the assessment of Christian biologist Dennis Venema, professor of biology at Trinity Western University in Langley, British Columbia, who forthrightly declared in chapter 3 of his 2017 book Adam and the Genome that it is scientifically impossible that the human lineage ever passed through a bottleneck of two, and we can be as certain about this fact as we are about the truth of heliocentrism.
Here’s Buggs’ take-down of the three methods employed by Venema to discredit the possibility of a single primal couple. As a layperson, I have to say it looks as if Buggs has done his homework:
Linkage disequilibrium within populations
…The methods assume that the populations at any given time point are at equilibrium and not expanding exponentially (the authors deliberately exclude the last 10,000 years from this analysis as they know that exponential population growth has occurred in this timeframe). It is hard to see how they could pick up on a short, sharp bottleneck even if one had happened. It would be nice to see this modelled, just to check.
…More recently, some simulations by a graduate student have shown that the PSMC method does not detect short, sharp bottlenecks, such as those caused by a pandemic or natural disaster. Thus I cannot see that PSMC analyses (many more of which have been done on human genomes since the original paper by Li and Durbin) can be cited as rigorously disproving a short, sharp bottleneck.
Incomplete lineage sorting
Venema makes an argument based on incomplete lineage sorting among humans, apes and gorillas, which gives a large estimated effective population size. This argument is not relevant if we are only interested in the human lineage (the occurrence of ILS does not require maintenance of large populations sizes in every lineage after speciation and so does not exclude a bottleneck in the exclusively human lineage).
We need to bear in mind that explosive population growth in humans has allowed many new mutations to rapidly accumulate in human populations (A. Keinan and A. G. Clark (2012) Science 336: 740-743). Hyper-variable loci like MHC genes or microsatellites have so many alleles that they seem to defy the idea of a single couple bottleneck until we consider that they have very rapid rates of evolution, and could have evolved very many alleles since a bottleneck.
In his conclusion, Buggs modestly refrains from claiming to have rebutted Venema’s arguments:
The question asked by my religious friends is different to the questions being asked in the studies discussed above. My religions (sic) friends are not asking me if it is probable that humans have ever passed through a bottleneck of two; they are asking me if it is possible. None of the studies above set out to explicitly test the hypothesis that humans could have passed through a single-couple bottleneck. This is what we need to nail this issue down…
If I am missing something, then I would very much like to know. Whilst this issue may seem trivial to many readers, for large numbers of religious believers in the world, this is a critical issue. Do they really face a binary choice between accepting mainstream science and believing that humans have, at some point in their history, all descended from a single couple? I am open to the possibility that they do face this dilemma, but I need more evidence before I am persuaded.
I would be interested to know what biologists think of Richard Buggs’ article. Is he right? Does science still leave open the possibility of Adam and Eve? Over to you.