Cross-posted from Evolution News January 9, 2020
This past October, Ola Hössjer and I published a paper, “A Single-Couple Human Origin Is Possible.” Writing in the journal BIO-Complexity, we described a model that used standard population genetics methods but refined in a new way to permit calculation of larger data arrays deeper in time. Using this model we were able to demonstrate that an initial couple could indeed give rise to the modern human population.
That paper discussed the possibility of a first couple, but it did not distinguish between two alternatives. The single couple could have had a de novo origin, meaning to start from the beginning. This alternative is one most scientists choose to ignore since it does not fit with methodological naturalism (MN), the philosophical position that only “natural” explanations are allowed in science. Natural explanations include only those things that can be detected, measured, or weighed using scientific equipment. Notice that MN automatically excludes intelligent design or any event like the de novo creation of anything. I noticed that after our paper was published, there was no discussion of the de novo option.
Instead, the discussion focused on the bottleneck scenario. A bottleneck means a sudden severe decrease in population size. It could be the result of a sudden population crash due to disease or environmental catastrophe, leaving only one male and female of breeding age behind or leaving them isolated from the group. Some objected that this was highly unrealistic because any disease so virulent or environment so deadly wouldn’t spare the couple either. Even if they did survive, the landscape would be uninhabitable. They would not be able to thrive and rapidly reproduce.
These would be valid criticisms if that were the scenario I had in mind. You see, it is not necessary that all members of the starting couple’s family of origin, whatever that may have been, died in a cataclysm. All that matters is that only the first couple had any progeny that made it through to the present. Most lineages die out over time; only a few survive to have progeny in the present. Seen from the present and looking backward, the lineages flow and join, as twigs do to branches and branches to the trunk. The phenomenon is known as coalescence. By definition, the first couple was the founding couple because the lineages all coalesced to them. There were no others from their generation who went on to produce progeny that made it to the present day. We are all descended from that first couple because all lineages coalesced to them.
Ultimately a coalescent event can’t tell you what went before. Was there a previous population, or was there no pre-existing population? Bottleneck to two, of de novo creation? These questions will have to be addressed by other methods.
Another point we made in our paper was the possible ages of the first couple. If one started with identical chromosomes, it would take 2 million years for natural mutational processes and recombination to reproduce modern genetic diversity. If we allowed for initial primordial variation, then that reduced the waiting time considerably, down to 500,000 years.
A Reference Frame
To give those numbers a reference frame, Homo erectus was emerging in Africa about 2 million years ago; Neanderthals and Denisovans appeared about 700-800 thousand years ago.
In our genomes, there are traces of DNA from Neanderthal and Denisovan DNA. The question is, how are they related to us? Are they human or brute? If the first couple is 700-800 thousand years or older, then Neanderthals and Denisovans could be their descendants also. I prefer the older age for the first couple precisely because that way, the hominids that follow them in time are human, and there is no need to invoke something abhorrent like bestiality to explain that small amount of Neanderthal sequence in our DNA.
Why Take Such a Position?
I bring this up because some have claimed that I advocate a sole genetic progenitorship model for the first couple. And by that they mean that I do not allow for any other genetic source but the first couple in our DNA. That isn’t very reasonable since the evidence belies it. Why would I take a position already shown to be false? I have always acknowledged the apparent admixture of Neanderthal and Denisovan DNA, and have argued that Neanderthals were fully human in the ways that matter. However, I do hold firm for one source for the human race, apart from that small admixture. There are competing models for human origin in the literature; thus far, it cannot be definitively decided which model is correct. I favor the single-origin out-of-Africa model.
I hope this short commentary will help to clarify and set the record straight. Finally, I need to end by saying these are my views and not those or my coworker, Ola, or of Discovery Institute.
The story of human origins is one we care about deeply. It touches on our identity — who we are and where we came from — and our most deeply held beliefs. Yet it is also shrouded in mystery and hidden in the deep past. We have minimal evidence from science, patchy at best, a sacred text with disputed interpretation, and no end of strife to go around. We would do better to lay down our arms and see what we can establish as true and what we cannot. We must also be prepared to give each other space for our beliefs.
Photo: An artist imagines a Neanderthal, from the Natural History Museum in London, by Paul Hudson, via Flickr (cropped).